@Yarideki There are at least two reasons I can think of:
Physical mask ROMs only came in fixed sizes, so unless you could manage to exactly fill up whichever size ROM you were using, there would always be some room left over; and
depending on how your code was written/compiled, you might want certain parts of your ROM to have fixed addresses for where they begin, either so you know code or graphics are always at certain places, or for working around any potential hardware quirks.