It's fetish dude. For some people, it's kinda arousing to see someone sucking
your finger, or doing that during sex
Mouth is a part of erogenous zones. Maybe it's just a coincidence that seme fingered uke's mouth after loosing his ass. Or maybe he think it's sexy to make uke tastes his ass. I don't know really.
geeluc is right, it's hot as fuck and such a turn on! fingers is the most sensitive part of the human body and the mouth is an erogenous zone (which is why kissing feels so damn good). makes all the sense! and making someone taste themselves is a fetish kinda
In some yaoi mangas, sometimes the seme inserts his finger inside the mouth of the uke and the uke sucks it. Like whats the point of doing that??? I really dont get it