Our dimmer switches (which are all slowly blowing out - maybe too much current? I don't know, I didn't install them) include this, but I've never seen it on other switches.

Weirdly some wall outlets have a little clear plastic section that looks like it should have a light behind it... but nothing. Asked a local 'builder' (I won't say electrician, because that doesn't exist here) and he had no idea what it's for either.