New style ground attachment
That particular switch is made for construction installers to do very fast work so they can get 2 houses done in 1 day.  "Time is money, friend". 

I imagine the source of your inquiry is that you notice how easy backstabs are to install, and you wonder why ground doesn't use backstabs too.  This is not allowed because backstabs are unreliable. Historically, NFPA and UL don't care if your circuit fails, as long as it fails safe.  They later realized backstabs do not -- backstabs are the reason that circuits now require AFCI breakers.  The backstabs make connection only on a tiny spring surface, this poor connection often causes arcing, which can lead to overheat, causing the plastic receptacle (and box) to char and burn.  The AFCI detects this arc and trips.  

The backstab situation is made worse when someone (who doesn't know any better) re-uses a backstab, which violates instructions (and thus NEC 110.3).  The spring is fatigued, and is even more unreliable!  

Most of us have had the experience of a scorched wire or melted socket from a failed backstab, and have sworn "Never again".  As DIYers, time isn't money -- time is safety.  It's no bother to use the screws.  Or we pay $2 extra for the higher tier Leviton ProGrade or competitor, which back-wire but clamp with the screws, making them a true lug instead of a hokey spring. 

That hole is to quickly wrap the ground screw - remember for best effect, wrap clockwise so tightening the screw tightens the shepherd's hook, not unwinds it.  You stick the wire through the hole, pull it over sharply, then clockwise around the ground screw, tighten the screw, done.   

Honestly, it looks better positioned to shape the shepherd's hooks for the other wires -- you'd wrap counterclockwise in that case, since you'd flip the wire over (nub up) to attach to the side screws.  Slick.