Does anyone know what the little slider under the toggle switch does? I read the manual for the device but see no mention of it… It seems unique to the dimmer as the On/Off switch doesn’t seem to have it.
That’s the “air gap” switch. It’s a way of temporarily disconnecting the switch from the circuit.
See the troubleshooting section on the following page. Many GE switches have this.
It’s the “don’t piss off the guests sleeping on the couch” switch. 
That is about the only time we ever flip that switch is during the holidays when we run out of rooms for visiting family. What I don’t like is that they don’t exist on the regular switches. Makes it difficult to disable the night time motion based automation and I end up getting the death glare from the wife.
Specifically, it’s so you can be sure the switch is off when changing the lightbulb, without having to throw the breaker and kill everything on the branch circuit.
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And you more often see them on dimmer switches rather than on simple on/off switches, because with the dimmer switch it can be harder to tell that there’s no current flowing to the fixture. It might just be that that the dim level is low.