From the systemd man page:
Use "${FOO}" as part of a word, or as a word of its own, on the command
line, in which case it will be
replaced by the value of the environment variable including
all whitespace it contains, resulting in a single argument.
Use "$FOO" as a separate word on the
command line, in which case it will be replaced
by the value of the environment variable split at
whitespace, resulting in zero or more arguments.
Since we want people to be able to use these for multiple arguments we
need to make sure we don't use {} around the env vars...
