Because CS-GO isn't a game to play on Max settings you want the Max framerates.
Why, you are limited by what your monitor can display.
I've heard that in Source games your mouse reacts different (better) when you are playing at a high FPS. Not sure if placebo or actually true. I'm not a game engineer. However, like you said, it's a technical limitation of the display, so basically 300 FPS should just display 144 on a 144 Hz display.