The big picture will always be a mystery. Why you fight is something else entirely.

Geoff Klock's Blog: Joss Whedon's Buffy and Angel: The Wide Angle View:

"Angel makes clear that in resisting evil we define ourselves, which is good for us, but has nothing to do with the cosmos. In order to fight evil, Whedon's characters, who don't call on the powers of good because there are none, harness evil to fight evil while trying not to lose their humanity..."