To a base registrations matter as all aircraft have different load limits , different seat config like WJAN as it has a row 1 a,b,c and other 75's dont in the tc fleet it also has no row 30 A,b,c and carries 2 less seats.
different take off weights BB has 108T limit and WJAN has 115T limit,
Plus for flight deck the ex chinese 757's have different fma's than the others
Also important for aircraft swaps ,maint,