No one is sure of the exact purpose of buck rubs. Yes, bucks will rub trees to remove their velvet, but with velvet-drop being a fairly quick process (24 hours or less in most cases), and since most rubs are made long after the bucks have lost their velvet, we have to conclude that rubs serve some other purpose or purposes.
From a hunting perspective, rubs can certainly indicate that a buck is in an area, and if the rub is on a particularly large tree, you can even determine that a given rub-maker is a trophy -- but don't assume a rub on a smaller tree was made by a small buck.
Whether rubs are used by bucks to mark territory or if they are used as signposts to line the route to and from a bedding area doesn't really matter to the hunter as long as he can use them to help determine where to hunt. When keying on rubs, look for a rub line -- or a series of rubs lining a trail. The rubs most often will be spaced 20 or more yards apart, but form a definite "line."
Rub lines are good indicators that a buck is hanging out in the area, and they also help keep your confidence up, which makes you a better hunter.