I changed a switch in my room with a Lutron dimmer. The switch was sharing the same receptacle as 2 other switches. I pulled it out of the wall and there were 5 wires coming out of it – red, white, two blacks, bare copper. The red and black are part of the switch and the second black is ganged to the other two switches to provide power. The copper is safety ground. Why is there an additional white wire (neutral) going to the switch? Note that it feeds directly into the switch housing and is not wrapped around any external screw. I ended up connecting this white wire to the pigtail with all of the other white neutrals since my dimmer didn't need it.
The other 2 switches in the same spot don't have this hole for the white wire and there isn't another switch for this light. All three switches are functioning properly.
I understand that some smart switches may need neutral to power in existing electronics but this was the original switch which has no intelligent capability.
Best Answer
You have correctly identified supply, and ground.
Now look closely at the legend (labeling) on the switch handle. See where it says "off" and if you flip it, "on"? No, you don't see that? That's because
It's a 3-way switch
The dead giveaway is the black screw on the supply being unlike the brass screw also in sight.
The other two wires (the brass screws) are travelers. When you throw the switch, it alternates which one is connected. As such, they are interchangeable, and I like to mark them both with the same color of tape, e.g. Yellow. In my world, two wires in a switch circuit, taped the same odd color, are always 3-way travelers.
There will be another 3-way switch somewhere, also with those same two wire colors going to its traveler terminals.
3-ways are complicated to add dimmers. In some locations Code requires both locations can turn on the light for safety reasons, and that fails if the dimmer at the far location is set to "very dim".
We would need to know more about your circuit and location to give further advice. This won't be the drop-in you were hoping for.