Off the top of my head:
1) The speed force nourishes the Flash so he can run almost indefinitely.
2) It can speed us his metabolism and thus his healing, allowing him to heal massive wounds.
3) It allows him to run without being affected by the air's resistance or raising winds when he runs fast. Of course some artists choose to show that he raises a wind, or even create a sonic boom when he breaks the sound barrier.
4) According to Einstein's theory, at the speeds the Flash reaches, his mass becomes huge (i think...) so at those speeds he can hit as hard as Superman.
And there's a lot more than that.