There are services other than bit.ly (like shorte.st and adf.ly) that stick interstitials between clicking the link and reaching the destination. So revenue. That's one reason.
How is that a benefit for the person using the link shortener service?
Say i have a facebook page with 100k followers. I can then say "check out this awesome page about ....", put it through one of those services. I'll then get $x per 1,000 clicks. Of course, my facebook page readers will have to sit through a second or two of adverts, but i'll get $$.
With adfly you (as the person creating the short link) get a cut of the revenue.