I know a national master that think the climate change is a hoax and that the earth is flat!
The consensus among coach is for any beginners to Think! carefully on Each move!
There's a point of diminishing returns when a player (especially a beginner) is given too much time to think.
Some (myself, in the past) might even reach a point where they become so overwhelmed by all the possible moves and lines that they have idea what to do. "Analysis paralysis", as the saying goes.
Having a shorter duration on the game clock can help with that.
Coaches have differing methods, for sure. Some never allow their students to play blitz. Some encourage it. Some never even allow their students to play at all (only study, analyze, do drills), until they reach a certain level of proficiency.
Keep in mind that when coaching beginners, often a lot of them are kids.
Blitz is a welcoming time format for young players, especially when it comes to casual games, against fellow students, for them to practice new ideas and techniques.
GM Allan Stig Rasmussen from Denmark, recommend players to play blitz to improve.
He must be the only one!
I took lessons from a National Master several years ago.
He recommended blitz to many of his students, especially the most timid ones, as he found that blitz forced them to make decisions at the board, taught them to value the initiative, and encouraged them to move past the hurdle of constantly second-guessing themselves, as "the ticking clock can't be pleaded with or reasoned with".
I had a GM recommend to me that blitz will improve my time management issues OTB (I'm very slow).
He was wrong, it was the opposite. I'm slow because I play blitz. When you never do classical time control length analysis, then once your in front of the board you'll calculate in circles not knowing what to look for.
In some cases blitz may be the answer, but in my case it was timed exercises. It could be tactical puzzles or endgame studies, but the exercise would be to decide on a move within __ minutes (I chose 5). So do all the analysis I wanted, but it MUST be over in 5 minutes or less.
That helped me set analysis goals and structure my thinking.
Timed exercises are good, too. Drills, puzzles.
But let's be fair to my former coach: he didn't say ONLY play blitz. Extensive study was still the main part of the process. Opening theory. Endgame drills. Positional puzzles. Standard games.
The blitz was encouraged to help develop the specific skills mentioned above.
IMO, blitz also helps with time management. Before playing blitz, I used to panic when the clock got low.
I drew an easily won ending (R+R+K+P vs. R+K+P) in my second tournament, because my clock got under a minute and I panicked.
Now a low clock doesn't bother me in the slightest.