They are so you can have all the config for a backend in the string you give to rclone. Very useful for people using rclone via the api to create backends on the fly.
It is also very useful for a little tweak to a backend that you want to apply to one backend only - this is my fave example from the docs
The major advantage to using the connection string style syntax is
that it only applies the the remote, not to all the remotes of that
type of the command line. A common confusion is this attempt to copy a
file shared on google drive to the normal drive which does not
work because the
--drive-shared-with-me flag applies to both the
source and the destination.
rclone copy --drive-shared-with-me gdrive:shared-file.txt gdrive:
However using the connection string syntax, this does work.
rclone copy "gdrive,shared_with_me:shared-file.txt" gdrive: