Reef knots were named for their use in sailing where they were used to tie down part of a sail during high wind. They are easy to untie which makes them good for that purpose, but not so good for uses where a more permanent binding is desired. According to Wikipedia, it is appropriate for tying up plastic garbage bags. Won't you sleep better tonight knowing that? Enjoy the peace of mind brought by having this useful knot diagrammed on your own unusual t-shirt.