Here's my theory of the modes.

In the middle ages, people in religious communities had an enormous amount of music to deal with. Music for the eight prayer services a day (matins, prime, compline and all that) music for masses, music for feasts, and perhaps for other things.

The musicians may have been selected at random or for reasons we wouldn't understand. Like that a brother has no musical talent, but he was born under a good sign for music. Or she's tone deaf, but her father was a musician by trade. A person could be chosen to sing because s/he has a beautiful voice, but the person could have been completely unschooled.

They probably had a little keyboard (organ played with a bellows), but who knows when we started naming the keys on them? (as in "D is here between the two black ones.") Maybe the keys went unnamed for a long time. Not only that, many people didn't even know their letters in order to read writing, much less to read music.

I think somebody invented the modes to help these poor musicians out. Trouble is, they don't help much. If somebody tells you that a certain chant is in the Aeolian mode, that tells you it probably starts on the key that's between the second and third black note (A). And it probably ends there. But where do you go from there?

It wasn't long before people started writing music down. It's a much better system, and the modes were more or less forgotten. In my opinion, it's fun when we encounter a modal song, but it doesn't actually help us master it sooner or better.