news flash: bands don't write their music so other people can play it.
different tunings do several things: first, they alter the tonal characteristics of the instrument. tuning one or all of the strings differently changes the tension on the strings and makes them react differently to being strummed. it also changes how they relate to any "static" resonances in the body, neck, pickups, or cavaties in the instrument. most bands these days are tuning lower than usual, which results in a thicker, beefier sound for the band as a whole, and also makes it easy to play in lower keys that a singer might find easier to sing in. the "1/2 step down tuning" in particular is used for this purpose. for some reason many singers find it much eaiser to sing to songs that 1/2 step lower, especially singers who are already singing pretty high (the weezer, green day, and dashboard confessional examples people have mentioned are all almost certainly the result of this). but it's important to realize that a normal guitar tuned to C or B (think smashing pumpkins' last album) sounds different than purpose-built "baritone" guitar (think staind).
second, they offer a different chord and melody palette to the player, and in some cases can facilitate playing things that are too difficult or impossible in a standard tuning. for rock in particular, dropped D and related tunings allow MUCH faster playing of various power chords, along with the ability to hammer on and pull off entire power chords at once. but many guitarist simply like the change of pace, since it forces them to play new stuff and get out of their ruts. I read Sonic Youth used to have a seperate guitar for each song, where the guitarist would twist the tuning pegs randomly, then write a song on the guitar. It forced him to always be original. The downside was he needed like 70 guitars.
But basically, IMO, if you're the type of player who is bothered by messing with tunings, you just shouldn't bother trying to play those songs. Playing other people's stuff is pointless anyway.