Some people like the idea of dividing even a small vanity with cabinet on one side and drawers on the other. Larger items can be stored in the cabinet and small items can be organized in the drawers. If you have small children, you can also place locks on the drawers used to store potentially dangerous items and cleaning substances.
Size is important too. It’s important for the bracket to properly support the shelf that it’s holding up. A common mistake is to buy brackets that are either to big or too small for the shelves. If they’re too big then they’ll stick out, and if they’re too small then the shelf could be unstable and dangerous. The projection of the bracket (the part that sticks out from the wall and holds the shelf) should extend to within an inch of the outer edge of the shelf.