After a long break from studying...mostly because of lazyness

I'm finally getting back to my ISCW studies.
Can anyone explain to me what the purpose or solution dialer interfaces provide? I'm trying to figure out why its necesarry or what advantage one would have with seperating a physical and logical interfaces only to tie them back together with a dialer pool?
Why not just assign the logical dialer configuration to the physical interface?
I know there must me an advantage of doing it this way, but i'm not sure what it would be. If someone could elaborate or give me a pratical example of its usefulness that would be great!