It's obviously important for a group to stay together. But staying together has nothing to do with a metronome. I've been singing with a large choral group (concerts usually in the Kennedy Center Concert Hall) for about 15 years. I don't believe I've ever seen our conductor use a metronome. Maybe he uses one at home as part of preparation. More likely he has an internal metronome--can visualize what tempo a piece should be taken at. I have seen him beat out quarter notes to give us an idea of tempo. But mainly he wants to be free to vary tempo.

Staying with the conductor ( or in another group, the lead instrument or the singer) is crucial. Keeping a steady beat is absolutely vital--so the group can stay together. But the group has to be sensitive to possible variations--which have to be determined by somebody acknowledged as the leader--it's obvious that everybody has to know who to follow. But again, this has nothing to do with a metronome.

If you want to know what a composer had in mind and there are metronome markings at the start of the piece, you can theoretically determine from them what he or she envisaged. But once you start, only if you wanted absolutely no variation in tempo in the whole piece would a metronome be useful.

I found some interesting stuff on use of metronomes. Beethoven was the first composer to use one--in 1817 he published metronome markings for all the movements of his (then) 8 symphonies. But the tempos indicated he indicated in some of his works (specifically the 9th symphony and the Hammerklavier sonata) are almost impossibly fast.
Of course in 1817 and after, his hearing was a real problem--I would guess this is probably a reason for the tempo problems. Interestingly enough, Schumann's works have the same problem.

I can't imagine that metronomes would be very useful in most types of folk music.