Another reason is on macOS where many file formats are actually directory based. (Called 'packages', they are actually structured directories with a metadata bit set.)
For example the RTFD format. This is a rich text document with embedded images. It is actually implemented as a folder containing a RTF file and the images. In the Mac GUI however it appears to be a regular file.
Attempting to upload it on the web would fail without the ability to upload directories. This may well explain why WebKit (being from Apple) was the first to implement this. I bet they needed it for their iCloud web app versions of Pages, Keynote etc.
