Its done in various forms on real ones to see how much ammo you have left, i know this isn't a thing in Airsoft but I'm a hardcore ADF LARP'er, and I would annoy my ADF mates if I didn't have that little detail.
I've spoken to an ADF chap I know who's used the F90 GBBR, first time i heard about it was about 8 months before it was announced to the world as he's been using them for the past few years.
I didn't believe him to begin with...
He says its great fun and wishes airsoft was legal there...