Of course Steve Vai is a famous user of the 7 string guitar, not so much these days perhaps. I think, like a 5 string bass, it’s for the extra low notes, so you can go down to a B. Therefore, I guess it would be popular with certain metal players who use that lower register on guitar, but still want the high notes for solos, etc. I also imagine a sweep picked arpeggio over 7 strings would sound rather effective.