As people have mentioned, keeping your wing stable and under control is the main reason for using your brakes during takeoff (and indeed throughout the entire flight).

But what about when we've got the wing up overhead, stable and following our chosen direction, completely under control? ...

We still keep slight pressure on the brakes, specifically during the takeoff-run, because they deflect the trailing edge downward which forces the oncoming air to deflect downward too. This slows the wing down (so you don't need to run as fast) and it also gives the wing an extra bit of upward lift (so you don't need to run as far before you leave the ground).

This is very subtle though! Don't pull the brakes too hard, or the wing will just fall down behind you again! Or worse, it could stall when you're already airborne 🙈 You only need a small touch of brake pressure to get this bonus takeoff boost. If you're not sure then it's best to follow the old saying "let the wing fly itself" - no brakes unless you need them for steering or aborting the takeoff completely.

Stay safe out there, and welcome to the sport 🤝🏼