I have had a couple of FWB relationships which were both caused largely by circumstances.
The first was with a girl I used to work in a pub with. This was in the old days where pubs had to close after lunchtime (2:30pm) and opened again at 5pm. When we were both working the lunchtime and early evening shifts she used to come to my parents house which was five minutes walk away rather than go back to hers which was a few miles. Having a couple of hours to kill and little in the way of entertainment (pre-facebook, DVD etc.) we soon started making our own entertainment in the conveniently empty house.
It was good fun and, as we were both recent graduates looking for career type jobs that would probably take us away from the area, neither of us took it too seriously. It lasted about two months before I got a job and moved away.
The second one was actually only a few months after this. I had moved to Reading and was living in a shared house with two other guys who worked days and a girl who was a student at the university. My job required me to work Saturdays so my “weekend” was Sunday-Monday. The student girl did not have any lectures on Mondays so we used to have a late breakfast and slob about. You can guess the rest.
Both of these FWB setups were largely as the result of circumstances and worked out ok.