High-quality or more expensive brushes have distinct advantages over the cheaper ones. First of all, a high-quality brush will finish the job more quickly. This is because a top-quality brush has the ability to "hold" more paint in reservoir, which means you will spend less time "painting the can" than applying the paint to the surface.
Also, a top-quality brush will have a tapered end, which means there are shorter bristles on the outside and longer bristles in the center. Tapered bristles give the painter more control over where and how much paint goes onto the surface.
A top-quality brush will also not shed bristles like a cheaper brush, because of how firmly the bristles are seated in the ferrule (the metal band that attaches the bristles to the handle).
The quality of a brush is also determined by the material used as plugs (space plugs inside the ferrule that bond the bristles in the ferrule, add taper to the bristles, and finally create "wells" in the center of the bristles to hold paint) in the ferrule.