Reefing a sail or tying a parcel, a reef knot the role will fulfill. But joining two ends one should only use bends, And a reef knot's a sure way to kill.
⸻Stuart Grainger, ex-Master Mariner, 1985. Referenced from "The Complete Book of Knots" by Geoffrey Budworth.
But since modern sailboats don't require knots for reefing, and modern parcels don't require tying, I think the use cases for a square knot are now pretty narrow.
Some times I use them to finish off some lashing because they can be easily tied while keeping tension, but that's about it. It's just not qualified for to be the first knot we teach a tenderfoot.