It makes a practical difference if you are playing a diatonic instrument (clarsach, pipes, whistle, moothie) - you need to know this stuff (or have an unverbalized instinct for it) when deciding which tunes to fit on which instrument and how.

There are other reasons, look at my modes pages.

The 19th century seven-mode pseudo-Greek system isn't all that useful for folk music, which is why I've described such an elaborate extension of it. (I didn't invent any of it - it's all ordinary parlance in ethnomusicology). I can see why "what's the point?" would be a typical response. Those names ARE a big so what - they don't even carry as much information as the eight-mode mediaeval system.

I really don't understand the jazz concept of mode enough to take that on as well (the ideas Lox was describing). It's a different system entirely - instead of applying to entire melodies, it's about moments within melodies (chords and scales used to play over them) - the melody doesn't need to have any mode (in my sense) when considered as a whole. Jazz is nearly always played on chromatic instruments and very rarely applies strict diatonic rules to entire melodies, as folk music and modal art music does. It's a pity that the jazz theorists didn't use a completely different word for it. A bit late now - we just have to work at avoiding misunderstandings.