what is object snap useful for?
Without them, you would have a lot of trouble doing even the simplest dimension accurately. Before discussing how they are used, first think about what they are. Their name, object snaps means that they allow you to snap to objects, or more precisely, you snap to specific parts of an object. A line for example has 3 points that you can snap to: a midpoint and two endpoints. A circle has 5 points: a center and 4 quadrants. To use Osnaps effectively, you must know what points an object has that you are able to snap to. There are also some Osnaps that are not particular points. These allow you to snap to (sometimes) arbitrary points along an object.