While you can do that with a Kobo you can't drop the file and expect it to appear in some collections. To associate a book with a collection you have to either use Calibre or the vendor's app. I have epubs and PDFs that weren't bought through the Kobo/Rakuten store and they don't get filed in my collections. With Calibre (and a plugin) I can do that.

I do agree I could live without it but files tend to accumulate and putting books in collections help.

All of this would be much easier if the kobo was dumber and just created shelves/collections based on folders.