They allow you to buffer everyone's playing, at a user specified interval, then replays the last measure of music to everyone.
It's definitely not the same as live playing, but it's still pretty fun, and actually forces you to get creative on different ways.
Big downside is you're stuck playing to a metronome, which would be enough for me to skip it, but it depends on the kind of music you're playing.
I could imagine that if the music is rhythmically slow and vague and improvised, big latencies are OK, and actually might yield some pretty interesting creative results.
Another model I've thought about is to structure players in a rooted DAG, and players can hear only people upstream of them.
E.g., you could build an orchestra by having a conductor and section leaders in a room together (or at within very low latency of each other). Other players could hear the leaders and play along, and then an audience could hear everyone. You could also do something more complicated like build things out in linear or power-of-2 layers, where each layer can hear everything upstream of it, and therefore many players would get a partial sense of the orchestral effect.
This could work nicely for improvised music, too, with causality preserved.
The weird side-effects are why I guess the parent said that "it's still pretty fun, and actually forces you to get creative on different ways."
It works pretty well with well structured music like the blues. You probably couldn't play well if the piece was changing tempo or key all the time.