Yes, automatic behaviour in one place, but there is a legitimate use for stuff moved into folders that are not linked to the server and that is, for want of a better word, archiving.
My IMAP server is at 40% of its quota. When I just had a PC, and used pop3, I could archive by any method, work/home, sender, subject, or just date.
Now I have a PC, an iPhone and an android tablet, whilst I am not going to go back and copy across all my emails from 2003 on all devices, but going forward I would like to do something like this. Currently I have immediate access on 3 platforms going back over 3years, and on PC alone ten.
Having drop box on all 3 platforms, the FILES could be shared, but the devil is in the detail - Outlook or Thunderbird is happy with 'local folders' that can be searched, viewed, sorted, printed etc, within the app just like live emails. Can't say the same about apple mail or aqua mail. No criticism intended. It is just that asking for folders outside the ones synced by IMAP, some people including me will be asking for these too to be synced by some means, but not within the mail server.