In case we forget what we bought and accidentally try to make a call off our version of the watch for commoners.

You can make a call from your non-cellular watch, provided you're in range of your iPhone.

Parent is slightly off in the details but the basic point is correct: the dot went from a filled circle in one generation to an open circle in the next. There's no functional reason for this, and the form of the crown itself didn't change enough for there to be any real reason other than for "spectators" to be able to distinguish the models visibly.

(And I'm not casting a stone here: I say this as someone who bought a Watch from each generation.)