1. Concentrate the wort to reach a desired post-boil gravity.
2. Reduce Di-Methyl Sulfide (DMS).
I made a Scottish 80/- last Sunday. I tried fly sparging instead of batch sparging, just to see how it would go. my extraction wasn't great, and I finally finished with a batch sparge to get some of the unextracted sugars out of the grain. I got all the sugars I needed, but I ended up with an extra 1.5 gallons of wort in the kettle. I boiled for about 2.5 hours in order to get down to my desired post boil gravity.
For a discussion of DMS see brad's article:
http://beersmith.com/blog/2012/04/10/dimethyl-sulfides-dms-in-home-brewed-beer/