for no particular or important reason.
I have no practical use for the Finnish language, but I'm studying it for the hell of it.
As an entire expression, this functions as an adverb. Sometimes "just" is added to the beginning to complete the expression.
1) For no reason, serving no higher meaning.
2) For pain.
3) "pushing the limits"; Testing God; solely for the purpose of sinning.
1) Even when I'm not tired, sometimes I sleep just for the hell of it.
2) I see her hurting herself often, and I wonder if she's doing it for the hell of it.
3) He sleeps with a different man's wife every night just for the hell of it.