Why should you need a power supply for preamp?

If you already have a power conditioner or regenerator? I mean it sounds like redundancy. For the cost of one of those preamp power supplies you could get a full system power conditioner or regenerator. I have a PS 1000 regenerator. I just bought a Bryston BP 25 with the smaller power supply to save money over the BP 26. Ive read there is little difference between the 25 and 26 anyway. Is my reasoning sound? No pun intended. Thanks all
Power supplies and power conditioners/regenerators are two different things.
The circuits in the preamp are not designed to run on 120VAC. The power supply provides the proper voltage level of DC power to operate the circuits.
The outboard power supply allows one to keep noisy transformers, rectifiers and the like outside and away from the preamp unit, which might have high gain circuits in it that might be quite sensitive to such noise.

An outboard supply is generally speaking a sign of a higher quality product.