'Lay back' seat posts? Why?

Why can you get some posts with huge lay back?

Are they for people who can't get their saddle far enough backwards or is there a comfort element?

 

They are to create a more stretched out position on the bike.

 

"CYNIC, We have a cynic in the house folks!
Please make your way out in an orderly fashion."

 

its for a very good reason. smaller frames are easier to throw about and handle better

 

If you have a relatively long body compared to your legs, you may need to sit further behind the BB to get a good balanced position compared to somebody more "normally" proportioned. If you use TT and stem length then you'll end-up with lots of weight on your arms, so some of us find layback posts pretty useful.
My TCR has quite a layback post as it has quite an upright Seat-tube. My Brooks saddle sits on the layback post almost as far back as poss. Stem length is 100mm, I'm very comfy!

 

Ah yes I get it now. Steep seat tube means further over BB = might need layback.

 

Also if you are tall and have a long thigh bone. It keeps your knee over the pedal spindle, instead of in front.