It tends not to. FWB is nice idea on paper.

“Hey we are friends. We are attracted to each other. We trust each other. Neither of us wants a relationship. Why not?”

Sure enough, in nearly every case I've experienced and heard of, there comes a point where one of you says some iteration of:

“So…do you think this could be something more?”

Friends with benefits typically ends one of two very different ways:

Losing a friend. (Most common)
A relationship.
Proceed with caution.