Dear modular,
This is the first attempt to describe what IMAP does that has made any sense to me.
My ISP recently changed its mail system and for a while its POP segment wasn't working -- and I've always deleted messages from my iPhone in hope of reducing the clutter. I was mystified when I re-did my iPhone mail as IMAP and after some weeks discovered that I wasn't getting about half my mail! And more so when I couldn't find the option to never delete from server (...'It was just here a minute ago...'
I would far rather take a chance on removing deleted inconsequential emails from my phone than face the certainty of losing emails vital to my existence as a business!
Who was it who first said "If you really understand it you can explain it simply" -- or conversely, "If you can't explain it simply, you don't understand it?" (Probably Moses, but Feynman comes to mind... and now Mr or Ms modular...)