Life isn't all work, and not all songs are work songs.

Often enough in a song session I'll sing without an instrumental accompaniment, even if I've got an instrument handy. Sometimes it's to save the bother of taking it out and making sure it's in tune, but often enough it's because I think it works better without an accompaniment. Or sometimes, if most other people are singing unaccomnpanied, it can seem more in keeping, and better manners.

The only reason to play an accompaniment is if it means you sound better, and do the song more justice.

All too often an accompaniment can take the edge off the singing and smooth out the variation in the notes.   You need to know a song very well and have sung it a good few times before it's likely that you will sing it as well with an accompaniment, even though you might find it easier keeping in tune with the accompaniment, if that's a problem.

Singing is a natural thing to do, there's no need to make a performance out it, as the saying goes.