Why are simplex channels designated in the VHF/UHF band plans?

Simplex communication means transmitting and receiving on the exact same frequency, completely bypassing repeaters. Band plans set aside specific simplex channels so that local stations can communicate directly without monopolizing a repeater. Repeaters are valuable resources designed to extend range for stations that cannot hear each other directly. If you and another station are close enough to communicate perfectly on a simplex channel, using a repeater ties up that wide-area system unnecessarily, preventing others who actually need the coverage from using it.