I'm guessing the regeneration powers of the unit would be a big help in battle. Zoaforms have to go back to the processing tank to heal faster, while the unit does it automatically.
Also, Guyot's gravity attacks were based on 'gravity points', little orbs throughout his body that were a sort of fuel. He and Imakarum expended almost all their points in order to do the black hole attack, and Guyot pointed out he only had the points in his knuckles left afterwards. The unit powers gravity through the gravity controller, and it doesn't need to be expended-so again you wouldn't need to go back to base and recharge. Don't get me wrong, after heavy gravity usage the unit needs a recharge period, but it's better than climbing inside a processing tank.
The self defense mode must also be a help. If an enemy gets a lucky shot in, it must be nice to have a copilot than can protect you until you get your act together.