Line connects to the wall socket - the phone LINE. The socket
labeled "phone" is a pass through connection so that you can still
connect up an actual phone to use when you aren't using the modem.
I am surprised, apparently few have used dial-up modems which allow
connecting through either modem jack. I seem to recall having more
than one modem which would allow that.
The likely explanation is that they use interchangeable metal plates
which are all printed/stamped for modems you describe. That is very
common, to use interchangeable parts. Also, I suppose a perfectionist
might get confused if he (or she) isn't told which jack to plug into
the wall.
For what it's worth. I was curious about something similar with
respect to speaker connections. Why do they have a plus sign and a
minus sign on the connections. After asking, I was told that
reversing speaker polarity doesn't cause electrical problems, but can
cause acoustic problems when using more than one speaker.