Because Y does changes to the defenders, like Press coverage and what not. So when you hit that the guy you are controlling is moved by the cpu AI defensive changes. When you hold both the LS and RS down, it won't run you off sides. Think of it as it's like charging your sprint, so as soon as the ball is hiked, you are going full speed, full acceleration.
Once you get used to it you can do Y and Left for Show Blitz, or Y and Up for Back off Coverage, while doing your turbo blitz.
Kinda hard to describe, but I hope that helps.