It's true that not all singing is "performance", but if the singing is a solo during which other people are expected to sit and listen atttentively for several minutes, doesn't that make it a performance?

If I "just sing", as I may do in the car or in the shower, it's not a performance because no one else is listening, so it doesn't matter if sing in a key that's too high, or forget the words and sing a lot of rubbish instead.

But if I sing at a singaround, and other people are listening to me, it's a performance, albeit an informal and small-scale one. I owe it to the other people there to present the song as well as I can. If I were to go into a singaround with the attitude that it didn't really matter what my song sounded like or whether anyone enjoyed it, this would be disrespectful and thoughtless to the others who were there.

As Hamish says, even people who haven't paid an entry fee will have set aside their time and probably incurred expenses for travel and refreshments. They have come out hoping for a good evening, and they deserve not to be bored or embarrassed. The least a singer can do is to choose and prepare their song carefully, and sing it as if every note, every word matters. Otherwise we'd all be better staying in to watch TV.