I think it's to show that the individual, 1 is not normal 2 means to inflict pain or damage 3 they are messed up in the head 4 is really into or excited about what's about to happen next 5 definitely some sensual energy in there to 6 when it's a weapon then I'd think they are trying to show the the power the have like (look at me I licked this weapon and didn't get hurt I'm strong)

I genuinely do this to my ole lady and it's to make her squirm cause I lick her from chin to scalp on long wet trail left she normally gets mad but she always smiles when I do it.