Sessions are for musicians - and singers are musicians just as much as anyone with a manufactured instrument, they are just using a different instrument. Just about all good sessions I have ever been to some of the time there'll be a person either singing or playing a tune on their own.

That may be because they are so good, or doing something so difficult, people prefer not to try to join in and risk spoiling it; sometimes it's because the person isn't sure enough of what they are doing to welcome the distraction of an accompaniment; sometimes it's because it's an unfamiliar tune, and the rest have the common sense to listen to it a bit before they feel ready to come in.

Sometimes it's a way of giving respect to a song - typically in an Irish session when someone with a small voice sings a sean nos song.

There's nothing "precious" in singing without an instrumental accompaniment - it's the first and most straightforward and natural way to sing, not some strange affectation. Yes, in most sessions there's an unspoken understanding that people are welcome to join in the music - but not if they've been asked not to.