Why I have one?
I play metal and death metal and metal core, mainly the really crazy stuff, being able to drop tune it into drop A and still have your high E string is a major gain.
Another thing is, I'm tired of tuning to different tunings, so if I leave it in drop A (A-E-A-D-G-B-E), I can just use a guitar capo to go to drop B and Drop C etc. It will sound about the same bc 7 strings usually have a longer scale length, mine has a 26.5 inch length (23.75" Les Paul).
I was always tuning between standard, D, C, B, And A all the time with my thick strings bc of the different songs I play for fun, but tuning down, you lose your higher notes so with an added string, you can still play pretty high with 24 Frets and your high E string.
A 7 string neck inst too hard to get used to or play. I played one for 2 hours at a guitar store and got so familiar so quick. It felt like drop A with another high E string. It will take a full week or two to fully get used to it, but with a lower end makes a wider view in playing.
It could also be useful such as on a Acoustic guitar for smooth full range barre chords, or for some of that Metallica slow-reggae-metal stuff, low strings and harmonics sound reallly good with that stuff. If you don't like the 7th string or use it every once and a wile, you could simply take it off, add a high A, or Ignore it and play the E string below it :)
It just depends on your style and role, go try one out and see for yourself, hope I helped! :)