Open A/Open D tuning (alternate tuning content)

[sublime0bass]

I have a friend in a metal band (Avow, Boone NC) who tunes to either "open A" or "open d" tuning, on his 5 string that is ADDDD

ive never heard of this before, what are the practical applications?

also, can i get some insight into how many of these alternate tunings work, and how they are formed/developed? I usually don't mess with them, as i like obscene string tension, but i am always willing to try new things.

[Mushroo]

That's a pretty unusual tuning (ADADA would be a little more common) but I guess it would be good if you play a lot of droning music in D! Another effect of it is that you'd need to move your left hand up and down the neck to play riffs. Perhaps your friend likes the feeling of moving up and down the neck rather than from string to string, and/or thinks it looks cool!

To answer your more general question, there are a lot of different reasons for using altered tunings: to play lower or higher notes than standard tuning, to use open strings in a particular key, to make certain fingerings easier, to get a different sound, to match what the guitarist is doing, etc.