I noticed that musicians often change their outfits during a concert. This applies not only to solo artists, but also to members of a band (at least lead singers).
Why is it important for them to vary the outfits during a gig?
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Musicians are entertainers, and a live show needs to have some incentives for the audience to show up. There are some genres --folk and grunge for example --where naturalness and authenticity are prized. It would be jarring to see a musician in one of those genres dress up in a fancy costume. But for a glitzy pop genre, the live show is a bit like a Broadway musical. The wild costumes and costume changes are part of the entertainment.
Sometimes it's part of the show they're trying to put on. Complicated productions tend to have a lot of changes throughout the duration of the gig, and clothing is just one of the possible aspects to play with to convey a feeling or intention.
Another possible reason is that it's usually really hot on stage, with all the movement and the bright lighting, so if the gig is long it's almost guaranteed that your clothes are going to be sweaty (I know this from experience as a gigging musician).