[Exim] Exim and LDAP

So - many people are using LDAP for a number of things. I don't really
know much of anything about it - but I'm begining to become aware that I
should. So - for those of you who are really into it - I have two questions:

1) Why do I want it?

2) Where do I start learning about it.?

My situation - I run multiple virtual domains using a linuxconf type
structure and I'm wanting to migrate into something with more of a
future. Not that what I'm doing is that bad - but I have this feeling
that LDAP is in my future.

I'm just not grasping the "big picture" of what LDAP is.