Originally Posted by dddd

It's especially strange in that no bike mechanic likely has a 4mm socket handy most of the time.

Conveniently, it is accessible with the plastic cap left in place. The 4mm hex extension seems prone to breaking if one perhaps tried to center the caliper with the mounting nut still tight(?).

Maybe the greater intention of the plastic cap was to prevent owners from trying to tighten the pivot nut as part of routine pre-ride check?
Such tightening might leave the pivot over-tight, with the owner unable to re-adjust things without actually having a proper 10mm thin wrench.

The knurl on the knob confuses some into thinking it's some kind of adjuster, which it is not.