1)Through the acronomic abbreviation of "(F)riend/s (W)ith (B)enefits" and the highly popular children's toy the "ferby" the term "FWerBy" came about.
2) The reason of fusing both terms was a "cuter and more playful associating" term for those involved in a "beneficial situation" since the 'ferby toy' is considered a 'substitute friend' for children.
FWB+ferby = FWerBy
Boy: So i guess i'm your friend with benefit now ei?
Girl: No silly, you're my FWerBy now!
*goes and 'play' with each other*