This is what is posted in the HD FAQ section:
Q: Is there any reason to play (or not play) an exhibition game?
A: Every game you play helps your players improve by giving them more game experience, exhibition games are no exception. In addition, it's also a great opportunity for coaches to experiment with different lineups, depths, tempos, etc. Those are the logical reasons, not so logical reasons may be that you want to take on a buddy in another conference or you want to see how your team stakes up with the elite programs in DI. The downside to playing an exhibition game is that you always risk having a player go down with an injury.