Sorry - didn’t see this until now.
**SEJ **- So, as most folks have discussed, with fretted instruments, de-tuning all strings a proportionate amount, so each is lowered one semi-tone is simply a choice - in effect, you have Standard Guitar Tuning a semi-tone or more lower. It would be no different than being able to stretch a flute before you play it, so the “base” tone with no keys depressed would be G, instead of A (or whatever it is).
We do it (and, yeah, I do it on pretty much all my electrics) for tone and feel. A lot of guitar players subscribe to the belief that the heavier gauge of string, the better the tone of your guitar. Some guitar players have a distorted enough signal that they can get a thick tone with thin-gauge strings (e.g., Jimmy Page) but if you prefer a somewhat-cleaner tone, string gauge is a real factor to your sound. Jazzers, who rarely bend, play with a .015 gauge on their high E string; metal players, who favor heavy distortion and a ton of bending, may play with .008’s; your average rocker will use .009’s or .010’s and Stevie Ray Vaughn legendarily used .013’s (going thinner towards the end of the tour) - his use of heavy gauge strings and his amazing tone (see post #21 in the linked thread)signalled a sea change in guitar circles when hair metal was king, with pointy guitars, locking whammy bars and thin strings.
But heavy-gauge strings are a bitch to bend - and blues and rock playing is all about bending. So as a matter of mechanics, tuning down lowers the tension of the string, making bends easier. I personally also like the feel when picking detuned, heavier gauge strings - they give good feedback, for want of a better term.
Add the fact that folks like Stevie Ray and David Lee Roth couldn’t hit the notes (neither can I ;)) and playing in Eb seems a logical choice.
As for your question in your OP - playing in E and A when using Standard Tuning just enables a player to take advantage of the physical characteristics of a stringed instrument. I typically play a Cheater’s A, where instead of using three fingers to fret the three notes required, I just use my index finger and flatten out the first joint to fret the same three note. That way I can use my other fingers to hammer-on single-note flourishes or even full new chords. And if you start in Cheater’s A, then use your middle finger to fret the G note (3rd fret) of the Low E, then hit the open Low E? That’s - well, that’s everything! I mean - starting in E, hitting that G note with your middle finger then hitting the A and then back down G to E is Muddy Waters’ Hoochie Coochie Man, Mannish Boy (youtube link - any questions?). That’s also AC/DC’s song TNT (youtube). That stuff is primal - playing in E or A unlocks the rock in a guitar.
You can also take advantage of drone strings in certain keys. The Edge and Billy Duffy of The Cult both made use of keeping the D string droning while playing partial chords up (and using a ton of delay) the neck for various hits - see She Sells Sanctuary.…
There are a LOT more techniques that take advantage of the mechanics of a guitar, but I gotta go into a meeting…