My question stems from ZZ Top's 'It's Only Love.' It has a slide riff, tab says the song is in B Major, the slide is Open E. WHY Open E, as opposed to Open B tuning? E shares four of the same sharps as B, is that why? Billy Gibbons liked Open E better than Open B, sure, maybe. But if an open tuning is close to a key, it'll work??
As @leftaroundabout said in his comment, you can use any open tuning for any key. Some will be a better fit for a key than others, but I'd say it mostly depends on personal preference, and how willing you are to retune with every key change (or how willing you are to carry multiple guitars and switch them around, I guess).
Personally, I rarely change tunings. I just find it too much of a hassle at home, and too disrupting on stage.
I sometimes dabble with open G, but mostly, I just use Open E, and use it regardless the key the song is in.
In addition, for me at least, it also automatically provides practice in getting a clean sound and muting unwanted resonance from other strings, by playing in any key that's not quite compatible with an open E.
Without restringing the guitar, B is really far away from the home pitch of all the strings. If you take the low E down to B it gets really flabby. And if you try to tune it up to B, then it's likely to snap.
E is related to B. It puts the root chord at the seventh fret and you have a little room to slide into the five chord at the second fret. Think of it like crossharp on a harmonica. It puts the root in a more convenient place.