Faldage:

There's not a musical difference between line and space notes; it's strictly graphical, as you pointed out.

However, when small children learn to read music, if they can recognize how the notehead appears on the line for line notes and how that same notehead appears to be caught in between the lines for the space notes, then they are on their way to reading music better.

Being able to state categorically whether a note is on a line or space leads to numerous, more highly complex notational reading skills, such as recognizing scale patterns, recognizing repeated notes, recognizing intervals, passing tones, chords, and so on.

If a child cannot even look at a note and tell whether it's on a line or space, that child is in deep trouble for ever being able to read notation.

Hope this makes sense. If not, just ask your question in a different way and I'll take a stab at it again.

Best regards,
DubDub