Thanks for your interest, Nairanvac. When I wrote that the full picture on altered tunings is kind of a "long story" for me, I meant a really "long story"! I'm sorry to say I don't think I can give a complete accounting of it here, not only because of the limitations of the format, but also because of my own limited abilities to organize my thoughts into words and my desire not to bore you to death with endless pontification! However, I'll do my best to answer your question with at least a Cliff Notes version.
It's true that for me the real advantage of altered tunings has to do with a quality I refer to as "resonance." I'm not using the exact dictionary definition here, but a related, personal meaning that involves harmony, acoustics, psycho-acoustics, music cognition and even perhaps psychology and anthropology. It's a very important concept to me in making the music I want to make.
I sometimes use the following idea to explain one of the ways altered tunings fit into my concept of resonance. I hope it will make sense: When you think of all the compelling pop and rock music out there that's been written in the keys of E minor, G major, D Major, A minor and so on, that's almost entirely because of the tuning of the guitar (and to some extent, the bass). The open strings just cause these instruments to resonate well in these tunings and produce sounds that lots of people find pleasing. Of course, it's easier to play the guitar in these tunings as opposed to, say, F major and Bb minor, but that's part of the point. In my opinion, pop music music has evolved in the way it has largely because there's just something very appealing about those sounds.
I'm not sure it's common knowledge, but the electric guitar and bass guitar are each capable of thousands of tunings and, as far as I'm concerned, each of these has its own qualities of resonance similar to, but distinct from those in standard tuning. I can't think of any reason why each one of these tunings isn't potentially capable of supporting equally satisfying entire genres of music based on their particular idiosyncrasies.
I think it's worth noting that whatever tuning you're working with, you're always dealing with the same 12 notes. At least theoretically, and perhaps practically, it's safe to assume that any piece can be played in any tuning. Why you would choose a particular tuning for a piece of music or visa versa, has to do with the kinds of phrasings and fingerings the tuning tends to generate and the way the notes sound in a particular configuration.
In working with altered tunings I use both intuitive and intellectual processes to try to understand, and tap into various resonances. In fact, altered tunings have been very helpful for me in the process of getting to know this concept of resonance in general, and in enriching my life by helping me more deeply experience the phenomenon of music.