if you are a beginner you will get the advantage when your opponents makes one move blunders or allows simple tactics. That's way bigger advantage then anything opening theory can give you. Also, beginners will not know the theory themselves, they will start playing the moves which you haven't expected, nor prepared on move 3 or 4. Opening theory is useless because it will never be used at a beginner level. And even if you actually play a line which you have studied, the advantage that you will get from it is nothing compared to the 1 move blunders and tactical blunders which both you and your opponent will make.
Below 900 blitz, 1100 rapid chess is all about 1 move blunders.
Maybe you don't think the same thing when you say chess beginner. If by beginner you mean 1500 rated players then I agree that it is beneficial for them to look into theory of the oppenings which they play and gradually add more theoretical stuff to their game through a lot of practice.
Beginner? Don't use this opening - It's too theoretical! Better yet, don't learn any openings!

if you are a beginner you will get the advantage when your opponents makes one move blunders or allows simple tactics. That's way bigger advantage then anything opening theory can give you. Also, beginners will not know the theory themselves, they will start playing the moves which you haven't expected, nor prepared on move 3 or 4. Opening theory is useless because it will never be used at a beginner level. And even if you actually play a line which you have studied, the advantage that you will get from it is nothing compared to the 1 move blunders and tactical blunders which both you and your opponent will make.
Below 900 blitz, 1100 rapid chess is all about 1 move blunders.
Maybe you don't think the same thing when you say chess beginner. If by beginner you mean 1500 rated players then I agree that it is beneficial for them to look into theory of the oppenings which they play and gradually add more theoretical stuff to their game through a lot of practice.
I would argue for beginners that want to improve that they at least know what opening they are playing, and common ideas that arise in the middlegame. I was stuck under 1000 uscf for five years. I'd develop all my pieces, my opponent would develop their pieces, and then we'd both be stumped as to what to do! I still have many of my old score sheets from seven years ago, nobody was hanging pieces! It might happen online, but it doesn't happen otb. And the stronger, 1000+ players? They'd have at least an inkling as to what to do.
There was this 1300 player who would fianchetto his bishop in a closed Italian game on b7/b2, then he'd get his other bishop on d3/d6, and then open up the position, and win!
I also played against this 1600 in a symmetrical Italian game. He sac'd his bishop on h3, and won five moves later. Meanwhile I was playing moves like h3, a3, b4, and then shuffling my pieces around aimlessly, hoping for "hanging pieces" because apparently, at my level, people "hang pieces" all the time.

people think learning the ruy lopez is like eating a hamburger, you eat it all in one sitting. Incorrect. It's like ordering a large pizza, and eating a little bit each day.
who says you can't eat a large pizza in one sitting
My dad doesn't let me
I played five, ten and fifteen minute games online. I guess that's the reason why there were more hanging pieces. What time control did you play over the board ? I wanted to give you advice to play oppenings which are not usual for your level (I started improving when I stopped playing Italian and Spanish and started playing b3 on move 1, and later the London system), but then I looked at your profile to see whether you have improved since then (because in that case it would be nonsensical for me to give you advice, since I myself am 1650 blitz, 1800 rapid player on this website and I have never played OTB). I see that you were rated 1200 two years ago and now you are 2200. May I ask who did you manage to make such an enormous improvement in only 2 years ?

he studied hard.
about 2 years ago (not on this account) I was rated about 400. after practicing tactics and watching plenty of chess videos I'm ~1400 now.

that being said gaining 1000 points is harder when you are higher. but just consistent practice should get you somewhere
I gained 600 points in first 6 months. Then I gained 500 points in the following 15 months. It is much easier to gain points when you are starting out. Going from 1200 to 2200 is a big deal. After you reach 1500 improvement gets really slow.

people think learning the ruy lopez is like eating a hamburger, you eat it all in one sitting. Incorrect. It's like ordering a large pizza, and eating a little bit each day.
Great analogy.
But there's a problem.
What if someone invites their friends to eat the whole pizza?
Get refuted more than the Jerome Gambit.

I played five, ten and fifteen minute games online. I guess that's the reason why there were more hanging pieces. What time control did you play over the board ? I wanted to give you advice to play oppenings which are not usual for your level (I started improving when I stopped playing Italian and Spanish and started playing b3 on move 1, and later the London system), but then I looked at your profile to see whether you have improved since then (because in that case it would be nonsensical for me to give you advice, since I myself am 1650 blitz, 1800 rapid player on this website and I have never played OTB). I see that you were rated 1200 two years ago and now you are 2200. May I ask who did you manage to make such an enormous improvement in only 2 years ?
otb i played no longer than 30min time control back then. After I made my account I've played more otb rapid (under 30mins) than actual classical chess.
And as for my improvement... I can't really put my finger on it, but eight hours a day didn't hurt.

I gained 600 points in first 6 months. Then I gained 500 points in the following 15 months. It is much easier to gain points when you are starting out. Going from 1200 to 2200 is a big deal. After you reach 1500 improvement gets really slow.
lol, I've been stuck at 2200 for a long time now...

I gained 600 points in first 6 months. Then I gained 500 points in the following 15 months. It is much easier to gain points when you are starting out. Going from 1200 to 2200 is a big deal. After you reach 1500 improvement gets really slow.
lol, I've been stuck at 2200 for a long time now...
hmmmm
i wonder how long it'll take for me to pass you
shut-

I gained 600 points in first 6 months. Then I gained 500 points in the following 15 months. It is much easier to gain points when you are starting out. Going from 1200 to 2200 is a big deal. After you reach 1500 improvement gets really slow.
lol, I've been stuck at 2200 for a long time now...
hmmmm
i wonder how long it'll take for me to pass you
shut-
i feel like i can pass you but it will take 4 months max
lmao
bet

and by pass, you have to actually pass me, you cant just one day get a higher rating than mine, your peak has to pass my peak

beginners really should focus and tactics, what's the point of getting a space advantage and a winning plan, if you blunder back rank mate?
would you really want to be:
The person who gets a winning position
or
The person who cheeses his opponent in a losing position?
a losing position with still a lot of pieces left is still playable if the two people are beginners.
people think learning the ruy lopez is like eating a hamburger, you eat it all in one sitting. Incorrect. It's like ordering a large pizza, and eating a little bit each day.