Faith is also a weapon modifier, increasing magical damage for weapons such as the Crescent Axe, Darkmoon Bow, Grant, Greatsword of Artorias (uncursed), and any weapon upgraded down the Divine or Occult paths.
Faith also increases Magic defense.
Almost every miracle has some level of faith requirement. Joining the Warrior of Sunlight covenant initially requires 25 faith (50 faith pre-patch 1.04/1.05), but the requirements can be lowered through cooperative play. For every boss that a player helps another defeat, that requirement will go down by 5 Faith.