I think it’s just an easy way of planning the TSC. Since the TSC in a way ‘distinguishes’ different signals(an thus help eliminate interference), it’s best to have a different TSC for cells near to each other. Since the BCC is already planned such that adjacent BTSs have different BCCs, it’s easy to plan the TSC by just equating it to the BCC. This will ensure that adjacent cells have different TSCs, and thus help minimize the effect of interference between adjacent cells.
This is not a scientific explanation, just my opinion.