I know you can create a file descriptor and redirect output to it. e.g.

exec 3<> /tmp/foo # open fd 3.
echo a >&3 # write to it
exec 3>&- # close fd 3.

But you can do the same thing without the file descriptor:

FILE=/tmp/foo
echo a > "$FILE"

I'm looking for a good example of when you would have to use an additional file descriptor.