The brass screw is for the hot connection, usually the black wire.; sometimes a red one. The silver colored screw is for the white wire or neutral. If there is a green screw, it's for the ground, usually a bare wire but may also be green.
Brass is relatively easy to cast, mold, machine, and is attractive to the on looker when compared to many other ferrous (iron,steel,magnetic) and non-ferrous (aluminium,copper,tin non-magnetic)...
Both screws are brass because in the US you need two hots to get 220V. In a 220V only circuit you do not connect the neutral, only two hots and a ground. This is why 220V breakers are twice as wide...