Something I've noted in Galactic Conquest, you get missions every so often and they tend to have various rewards, generally new units that you normally wouldn't be able to build that can supplement your own forces. There's also a second reward, or repercussion in the case that you fail to complete the mission in time, is senate approval. Finishing or failing to do a mission will adjust the senate approval by either +1 or -1 however I'm not sure what that does overall.