(Almost) any hull form moving through water will see hydrodynamic forces acting upon it to cause movement in a direction that is not aligned with the parallel axis of the boat. The crazier the hull, the more you'll see evidence of these forces. A modern nuclear boat... pretty small. An old fleet boat, or a fantasy boat like the Nautilus from 20,000 Leagues or the SeaView?... oh yeah! Big time!.
Without planes to counteract these forces, no amount of precision trimming via your ballast system will be able to keep the boat at a stable depth. Additionally, ballast system can rarely react quickly enough to counteract these forces. That's where the planes come in. The other thing the planes do is allow for pitch control of the boat, giving you the ability to "crash" dive or surface quickly, or counteract these off-balancing forces that want to pitch the boat off its center line.
A nicely trimmed boat, tanks vented to neutral buoyancy, and with some decent control surfaces to effect change, is a beautiful thing to operate. Get your trim wrong or have insufficient surface area or throw on your planes and it can be like wrestling a snake...