you have an architecture something like this
External N/w GMSC MSC BSC BTS < Air interface > MS.
I have not shown some other blocks here for simplicity.
BTS acts as an interface between your MS and your BSC. As MSs are addressed over the air, you require BTS to address the MS through RF energy. Added to that BTS output power also signifies how far over the air you can reach the MS.
(I think I have tried to be as simple as possible)