There are several reasons a guy will offer to remain friends after a breakup.
1) He still cares about you and really wants to be friends. You see this a lot with long-term relationships that come to an end. The couple has been together so long, they can't imagine not being in each other's lives in some capacity. These are the rare instances in which friendship after a breakup can actually work.
2) He wants to lessen the blow because he feels guilty about hurting you. The offer of friendship may or may not be sincere, but it won't work, anyway, because guilt and pity are poor foundations for a friendship.
3) He wants to keep you on the line as a fallback in case he has no luck finding anyone else to date, or he's keeping you around for sex until he finds someone else to sleep with. Don't be the woman who falls for this.
It all depends on the guy. Some of us have a conscience; others don't. Reasons two and three are why I continue to believe that clean breaks are the best choice 99% of the time.
Thanks for the question.