You could ask the same question about piano! The thing is, a "normal", "standard" guitar is tuned EADGBE. That is how they are designed and built to be tuned. To me, there are only two reasons to deviate from that:
1/ Because it allows something to be played that would either be impossible or very difficult in standard tuning, because of the limitations of the guitar/tuning NOT because of the limitations of the player! An open tuning is a good example (especially when a slide is being used) and so are DADGAD and dropped D. In fact, probably every alternative or altered tuning used by competent players (all of whom are way better than I am) is fine by me.
2/ Because of a physical disability. Your example of Tony Iommi is a good one. Anything that allows a person to continue to play after an accident or start playing despite a disability is a great thing. But I don't count stupidity as a disability!
I guess that years ago lack of information would be another issue. Apparently when McCartney met Lennon he (Lennon) had his guitar tuned "incorrectly" and had made up his own chord fingerings; apparently Richie Havens had no idea how to tune his first guitar and arrived at an open tuning using his knowledge of singing in church.
The thing is though, the idiots that post questions here don't fit into these categories. They are the same people that don't know how to use a capo, think the truss rod is adjusted to alter the action of a guitar and think that you "learn guitar" by learning "songs" from tablature (or "tabs" as they call it). They think a guitar is tuned according to the song being played and that a chord is named according to where you put your fingers rather than the notes it contains, so the basic nut-position Am chord shape is Am regardless of how the guitar is tuned. Over the last few weeks we've had questions from one bass player who was using various "drop tunings" to try to match the tuning of the "guitarist" (did it give him any extra notes?) and another five string bass player who was tuning everything except the fifth string UP. These are people who simply haven't got a clue: they know nothing about playing the guitar and are unbelievably ignorant about everything to do with guitars and music. And, yes, they like to use a "drop tuning" so they can play one-finger power chords! Some of the questions posted here about de-tuning guitars are really just too ridiculous to answer and the only thing to say really is, tune your guitar properly and learn the basics".
Some of my favourite guitarists are: John "Charlie" Whitney of the British band Family who did a lot of lap steel work in open tunings (and used a capo a lot), Richard Thompson who uses various tunings, especially on a acoustic, and favours dropped D a lot, not because of the one-finger power chords but because a lot of his stuff is in D and it allows a fuller bass end, Simon Nicol of Fairport Convention who, again, uses a lot of dropped D, and Leo Kottke who seems to use a lot of alternative and altered tunings.
I have one electric guitar permanently tuned to open G for slide, one acoustic whose tuning various but is often DADGAD and I often use dropped D on acoustic. Years ago a band I was in covered Question by The Moody Blues for which I tuned to open C.
To me, any competent player who has at least mastered the basics will use any tuning or tunings they want. A standard, ordinary guitar though is correctly tuned EADGBE - that's why it's called "standard tuning" . If someone asks, how is a guitar tuned? The answer is EADGBE. To me, 1 and 2 above are the only good reasons for deviating from that.
Edit: most of the people posting questions here about tunings aren't using alternative tunings really anyway, they are simply de-tuning the instrument which acheives nothing.
@PatrickM you see, to me, that's a good example of a competent guitar player who knows what they are doing tuning up or down, by ONE semi-tone, for a good reason that makes sense to me. Not long ago we had a person wanting to de-tune by six semi-tones and an answer from someone saying that, yes, that was fine, didn't even need to use heavy strings!