Coach is one of the biggest car driving on roads of GTA3 era games. It is the only vehicle with no transparent windows. It has unique entering with special animation - Driver enters same door as passengers, and after few seconds we are ready to drive. It's apperance changed slightly in each GTA.
It is one of the heaviest and probably the longest vehicle in GTA, thus it has very big turning radius. It is quite fast as for a truck, and got quite useful handbrake. It isn't as durable as other trucks.
Although no important mission is connected with Coach, but it is very useful when need to block some narrow road. There is no bus mission in it, but passengers still enters and exits it at stops.