Unicom on VATSIM is 122.8 and is used for pilots to communicate with one another when ATC is offline, or you are in uncontrolled airspace / non towered, etc…
For example, if I am flying into Toronto Pearson (CYYZ) and there is no ATC, I would communicate my intensions on UNICOM and if there are other planes in the area that may conflict, we’d coordinate together to get in.
“Toronto Pearson Traffic Air Canada 121 is 100NM north of the field at FL340 and will be starting our descent via the AVSEP7 arrival MUSIT transition expecting the ILS 06R”
If another pilot here’s that, and it may conflict with their approach or departure, they could communicate with me so we could coordinate and ensure everyone gets to where they need to be.
That is just 1 of a 1,000 examples.
Non towered fields, flying a circuit would be another.
“Timmins Traffic Cessna C62SKP taking off runway 03 remaining in the pattern, Timmins”.
If you google “VATSIM UNICOM” there is a ton of information.