I can't see what's in it for the performer, I can see what's in it for the receiever. I'd be curious to find out why someone would be willing to give without receiving.
Some guys (and a few women, too, but far fewer women than men, in my experience) just enjoy the taste and the sensation of sucking a guy's cock and having him ejaculate in their mouths. And some people are just generous.
I used to see a guy regularly who was just like that. He was a really cute, studly, guy, too -- but all he wanted to do was suck
my cock. Once, I noticed that he was hard -- and he is even better hung than I am -- and offered to reciprocate, but he declined. And though I've cum in his mouth in all states of dress and undress (from naked, to completely clothed with just my cock poking from my jeans), depending on how far he wants to strip me and how much I want to take off,
he's never so much as unbuttoned his shirt.
I used to wonder about him, and whether he was really satisifed, but I quickly moved to just enjoying his services. He's just about the best cocksucker I know, and that's the highest praise from me.
I should go visit him after work this Saturday...
(And yes, I found him on Craigslist!)