Subclasses present a new way of augmenting your character into a specific role beyond that of a normal character class or by obtaining prestige class levels.
Everyone has a natural gift. For some people it's a pleasant singing voice or a good memory, but for the marksman it's a talent with non-thrown ranged weapons. She can catch a fly on the wing with a slung stone, or shoot an apple off your head and land it in the mouth of a roast pig halfway across the room.
Some marksmen hone this natural talent to a high degree; gaining a level of skill that makes even the most legendary elven archer green with envy.