I think the main reasons I like singarounds are:

The opportunity to sing, both alone when it's my turn and with others when there's a chorus.
The surprise element of not knowing who's going to turn up, and who's going to delight you with a great new song (or a great old song).
The in-between-songs banter and crack - singing creates a bond among the singers, which fosters social contacts and friendships.There's also a chance to give and receive some informal feedback about the various songs and performances.

All of this can, of course, fail to materialise at the very worst singarounds, but on most occasions I find I can tick off all three of the above benefits.

Marje