Singarounds are first and foremost about joining in. That's what makes them so enjoyable. It is also the major reason they are different from concerts.

You are not forced to join in, but the MC at a singaround will usually ask you if you want to take your turn. Even if you don't want to or don't feel confident about singing when it's your turn, you can always join in the choruses. Someone said they prefer sessions because you can join in quietly if you are not very confident (or something like that). You can do the same with choruses.

I also enjoy sessions because, like singarounds, they are about joining in.

Both singarounds and sessions are social occasions where people get together to share a common interest.