Earlier quoted context omitted.
"I can't think of a single reason why Apple would want to delete the files from the user's computer..." Well, to save precious local storage space of course. Apple's just being helpful here. But seriously, sounds like a "good idea" gone wrong - "I know! Let's save the user some space by serving all their music from the cloud!" The moment that file deletion came up in the design, the option to opt-in should have been…
The funny thing is: Disk space is only worth more than bandwidth in a few select 1st-world countries. I've been traveling in rural areas of South America for about 18 months now and regularly I had to turn off my MacBook because its automatic update "features" were consuming all of the available bandwidth, making it impossible for anybody to actually work.
Which automatic update features, by the way? While I definitely don't like or defend such features, all the ones that I know about can be turned off—which gives a more useable result than having to turn off the whole computer! Maybe my Mac is silently doing some behind-the-scenes updating that I haven't noticed hitting my data, though.
(Windows 10 is the real pig here, though. Bye-bye, option to download only on request! Bye-bye, this month's data allowance before I realised that it was downloading what turned out to be a huge update! I did eventually discover how to designate a connection as 'metred', a nice feature that I wish Mac OS had.)