It allows you to make smoother turns, try releasing the accelerate button and holding slide as you take a tight corner - it's super important on some of the later tracks.
ALmost like a drift mechanic for sharp turns
Actually, you don't have to release the accelerate button.
I was able to slide just fine with both buttons pressed.
Thanks for the answers :)
It seems I still need practicing though to really understand how to best use it ahah XD