This is due to frequency re-use in a cellular network.
The BSIC is defined in GSM specification 03.03 section 4.3.2.
The NCC of different providers must be different; they must also be different in national border areas.
The broadcast control channel (BCCH) of each base station is assigned a frequency by the network operator, and they must be assigned such that no neighbor stations have equal BCCH and thus equal BSIC.
Unique identification of a base station is especially important in border areas, where at both sides of the border there is a different operator who might use the same broadcasting channel on the same frequency.