This topic has been discussed before. FS is often just a physical act: positions 1, 2, 3...shoot. A BS can go beyond the physical into the fantastical. It's the fantasy type of experience that many guys who prefer a BS over FS go for. Even though both are still a service at least perceptively a BS can be more intimate than FS. The MPA's that give the best BS have learned the art of seduction and sensuality that most that only offer FS haven't.
Hey, some guys want a nice liquer or premium drink at a fine restaurant where other guys want to chug back some beers at the local bar. After about the same amount of money spent the end result is the same but the journey is very different.