The only difference between a Half Hitch and a Simple Overhand Knot (or Simple Knot) is the fact that the rope is attached ("hitched") to something (like a pole or another part of a rope.) It's called a "half" hitch, because two of them are needed to hold any load. Using just a half hitch can be dangerous and needs another half hitch to be considered whole.
By itself, this is not often a useful knot, but it can be the basis of many others. As such, it makes a great introductory knot. Those new to knot tying may be comfortable with this knot and then work they way up to others.