First use of Desktop & Documents option for iCloud Drive

I think I'll enable the option now that I've figured out that timemachine is capable of handling the backup of different versions over time.

My question is: I already have a lot of files on my desktop and in my document folder.

How will the option behave (basically will it take my existing content and make it the content that will go on iCloud).

Now that I've written my question, it seems pretty stupid but I'd rather make sure before (even if I don't see our dear apple deleting my local data).

Thank you.