If you're not involved with the Virginia housing industry nor morbidly obsessed with it, you might not be familiar with proffers, which some Virginia localities collect to help offset the costs of new development: more roads, bigger schools, etc. In exchange for rezoning approval, developers often agree to provide land for road construction, or make stormwater improvements, and so on. In some cases they simply offer cash. The theory behind the proffer system is that while ordinary taxes pay for operating costs, the locality needs something extra to pay for capital needs like libraries and fire stations.
In theory, proffers are voluntary. But as A. Barton Hinkle observes, in practice they're about as voluntary as the money you fork over to a tow-truck company to get your car out of the impound lot.