Compensating bridges compensate for the fact that different thicknesses of strings bend in different ways when they're fretted, and also stretching them when you press them down to the fingerboard produces an unwanted small change in the pitch from what you'd theoretically expect. Straight bridges placed by ear (compare fretted octaves with harmonics) usually give a good usable compromise, but some banjos seem to need a bit of compensation in the shape of the bridge. If you can't hear it, you don't need it.
Cheers
Ray