Random thought - why is there a sensor on the rotate axis?

[Mike]

Why did they bother putting a position reference sensor on the rotate axis?

The only thing that matters is the relative rotation between the pick & the place (maybe also the tool pickup if it's an orientation-specific tool, like melf).
So why the need for an absolute rotation reference (and the annoying pause while it resets it) ?
 

[phonoplug]

...so it can correct for runout on the rotational axis.

not sure if it actually does a correction, but it has the ability.