Confusion in spanish, marshall gambit

It's just a different variation.
The main starting position is after 14...Qh3, but along the way both black and white have a few different options... a real nightmare for an amateur to be well prepared. If you don't mind that sort of thing though, you can get some nice wins as black... which is why when black castles instead of d6 (signaling a possible marshall) it's recommended to play 8.a4 or 8.d3 as white to avoid it.

Bb7 does not defend the same as c6. You can't play Bd6 because you hang the knight.
Ohhh. Mhm I knew I was missing out and being oblivious to something simple. Now I see, so in this variation black cant stay as aggressive in a way. I just noticed the bishop takes on h2 discover attack, if white ever initiates exchange on d4 only because you drew my attention to it. Thanks!!!

It's just a different variation.
The main starting position is after 14...Qh3, but along the way both black and white have a few different options... a real nightmare for an amateur to be well prepared. If you don't mind that sort of thing though, you can get some nice wins as black... which is why when black castles instead of d6 (signaling a possible marshall) it's recommended to play 8.a4 or 8.d3 as white to avoid it.
I had understood to try not to get into it as white. I see that it really does require a lot of preparation and it is not to be played carelessly just because I can memorize the first couple of sequences. Thanks for the insights!!!
The Marshall gambit is one of those openings that's played for very different reasons by amateurs than it is by GMs. Amateurs get initiative and attacking ideas... like Pfren said this is strategically straightforward. Black has good winning chances, and white does better to avoid it at lower levels.
GMs, with their immense theory, play it for a draw... and white avoids it in search of better winning chances.
MVL did beat Svidler with the 12.d3 line in their recent match... it wasn't because he got anything out of the opening though.

In the original famous game from 1918 Marshall's attack looks absolutely devastating but Capablanca defended like the legend he was. If you are thinking of playing it bear in mind that it wont be one of the greatest players of all time facing you with the white pieces. Here it is.

The point is not to mimic great players blindly but to take inspiration and enjoyment from the great games they played. Most of us are here for fun and hopefully to improve, not to become titled professional chess players. If the OP is interested in the opening he should play it. Understanding comes from doing it, making mistakes and learning from them.