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I would say that the most important aspect is conveying what the band/artist represent, to give an idea to the audience of what to expect and in some instances to show the dynamic of the band.
I personally love the music of Spangle Call Lilli Line (pic related) and I feel like their band photo represents their sound quite well. Calm and smooth light blue hue. Obscured faces, keeping a level of privacy, not trying to advertise their looks but their work.
In this case I'm completely biased though.