By fuckboy I assume you are talking about a male slut? For the same reasons sluts have boyfriends. They enjoy having sex with that person, need the companionship, or are using them for financial security. There may be other reasons but those are the most common.
Most Helpful Girl
Fuckboys usually don't unless they feel like some down time. Even slutty guys want love now and then, but don't hold your breath on being that girl.
Players play for power. They enjoy the mind games and a partner makes a fun toy for them.