It lets Sirius pretend there's something more between them than sex.
I like that element that usually comes out in Snack (particularly teenage Snack) -- Severus is resigned to the "fact" that all Sirius wants his a place to put his cock and feel superior, and Sirius turns out to actually be harbouring at least a spark of something much more gentle. (And then manages to push Severus away regardless, and goes "Oh bugger. ...Pun not intended.")
I like that element that usually comes out in Snack (particularly teenage Snack) -- Severus is resigned to the "fact" that all Sirius wants his a place to put his cock and feel superior, and Sirius turns out to actually be harbouring at least a spark of something much more gentle. (And then manages to push Severus away regardless, and goes "Oh bugger. ...Pun not intended.")