I keep a few of those Home Depot medallions on my truck, they work well.
A lot of the old fans that I take down have a much larger canopy. When you put up a new fan with a standard sized canopy there is a large unfinished and unpainted ring of bare plaster around it. If the customer doesn't want to worry about finishing and painting it, I am happy to sell them the medallion for a markup and installation fee
Also, I have taken down more than one old fan that was installed over a 6" recessed light :lol:
Finally, sometimes the old light fixture was the only thing holding the plaster up there :rant: So these medallions are definitely handy.