I recently changed my sons' bedroom light switch with Lutron smart dimmer. The house is fairly new and has not been remodeled. I have changed out about 3 other light switches without issue. This particular switch had an addition wire connected. I thought that maybe it was traveler so just didn't reconnect it when installing the smart switch. Turned on the power and the switch works well however, the receptacles (only in this room though the breaker runs 3 rooms) no longer work! What could this wire be? Thank you in advanced for your help.
Without KNOWING how it was connected (which it's always good to document before you tear stuff apart, but too late for that) I'm going to GUESS that the outlets were getting their power by being joined to the unswitched hot feed to the switch.
It happens all the time where a device is used for a splice, and the splice is unrelated to the device. So it seems inexplicable.
Let me pause and say the word "outlet" does not mean only receptacles, but any point-of-use including a light/switch combo.
As you certainly do know, there isn't a home run back to the panel for every outlet. Most outlets also have power being chained forward to the next outlet. Receptacles have extra screws just for this purpose (actually for dual purpose, but this is one of them).
In a switch, you have neutral coming from supply and carried onward both to the lamp and to the next outlet. An old style switch doesn't need neutral, so this 3-wire splice happens in the back of the box at a wire nut.
In a switch, you have always-hot coming from supply and carried onward both to the switch and to the next outlet. That's 2 wires + the switch. You can do that with a pigtail from the switch combining with the 2 wires (making 3 wires, almost exactly like above). However some installers just put the other 2 wires on the switch itself, using the switch as an ad-hoc splice also.
I recommend converting all such setups to the pigtail arrangement. First, it's easier to understand. Second, most smart switches have pigtails, so you'll have to do it anyway. And third, switches aren't intended to be splice points, but they have both screw and backstab. This splice trick typically uses the backstab, and backstabs cause nothing but problems - mostly open connections (and they're extra hard to find when they're at a switch), but also arcing, meltdown and fires.