As you work your way through (ie. beat the hard level races) you will open up more manufacturers and tracks.
Ultimately you open up the two in-game moves (one set to a classic piano piece, the other to a Lenny Kravitz song).
You also get a sizeable chunk off your game completed percentage.
If you beat the hard race first, it counts as the easy and normal difficulty as well, so you only need to beat the race once if you beat it on hard level.
Also, in arcade mode the time trials count for the percentage completed - and if you get gold in all of them you win a nice Panoz Esperante - it looks and goes like hell!