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.
One who travels to distant lands to fight for their deity against infidels in contested territories is a crusader. Most journey toward designated holy lands to fulfill religious duty or perhaps as penitence for sins. Many may receive a calling from their deity from priests or from a direct vision. Despite their personal reasons, a knight takes up arms against those in the holy lands. Most crusaders are often spurred towards such ends by charismatic church leaders.
Rewards for success could earn a crusader reputation, redemption, and possibly a tidy profit. However, most crusaders look toward eternal reward with their patron god or goddess. These faithful warriors take great care in dispensing the full fury of their faith upon those considered infidels and heretics.