I am confused by what KMail’s option “server-side subscriptions” does or does not and whether enabling or disabling it may cause harm.
Today I have re-arranged my IMAP folder structure (because it too was confusing me, but that’s a different story) and renamed several folders as well.
To my surprise, entire trees of subfolders suddenly vanished, leaving only top-level folders, after I had renamed the top-level folder.
With a different mail client I could confirm that the sub-folders were not lost (oh my dear). Then it occurred to me that I had had a similar situation a few years ago and that folder subscriptions were the solution. The “Manage local folder subscriptions” command from the folder list context menu did not help me, but I did rediscover the option “Enable server-side subscriptions” that is shown in the screenshot above.
Clicking on the butten “Serverside Subscriptions” revealed that entire trees of sub-folders had been deselected. Instead of clicking and re-enabling numerous subfolders, I tried what happened when I disable the option.
With server-side subscriptions disabled, all my subfolders reappeared 
But now KMail apparently started to refetch ALL emails from the IMAP server, that’s several GB. 
My questions are:
- What is the benefit of enabling server-side subscriptions?
- What is the benefit of not enabling them, other than my folders re-appearing?
- Are there any important drawbacks to consider, other than several GB of network traffic?
Thanks!
P.S. For cross-reference, I’ve found these similar, but distinct topics: