From https://unix.stackexchange.com/questions/402555/why-add-a-trailing-slash-after-an-rsync-destination
It does make a difference when the source is a file and the destination directory does not exist. For instance take a file called file as source:
$ rsync file dest/ will create a copy of file inside a directory dest, whereas
$ rsync file dest will make a copy of the file file called dest
To add (from the comments); if a directory dest already exists, a copy file will be created in dest in both cases above.