Boil Volume

I've been learning a lot about all grain mashes by reading John Palmers book and still have one question I can't find the answer to. Why do a lot of recipes for 5 gallons require 6 or 7 gallon boils? What do you do with the extra gallon or two of wort? I know there will be some volume of it that boils off, but it seems like a waste if you just pour the rest out...

 

One factor is that it depends on a 60 to a 90 minute boil.

 

I start most of my boils with 7g, and boil for 75 minutes. (Bring to boil, wait for hot break, wait an additional 15 minutes, add hops, and boil for 60 minutes).
After the boil, I transfer to the fermenter via a CFC, and end up with between 5 and 5.25g in the fermenter. Evaporation accounts for most of the loss, but cooling, hop absorption (I use dry hops), trub, and dead space in equipment also plays a part.
For IPA's (with a large amount of hops), I need to increase the volume by about a qt. For lagers based on Pilsner malt, I need to increase the volume even more because I do a 90 minute boil after the break.
I don't throw any wort away. It just disappears.

-a.

 

Thanks to everyone for all the info, I guess I was just underestimating the amount of boil off. I guess this applies to extract also, I just haven't noticed because I've always had to do partial boils and add at the end. Finally I got a big enough pot to do full boils, so I'm probably still quite a few batches away from going AG, but I was just curious.

Cheers!