It makes sense if you want to concatenate strings, which is probably pretty much what happens when you type that into the path input.

This way, you could create a string like "D:/%appdata%/local/minecraft" Whereas without the trailing %, you would need a space to separate it from the text, which would result in "D:/Users/Public/AppData /local/minecraft".

(i havent used windows in a while and those paths are from memory, they might be wrong)