Higher grade bikes are lighter and usually stronger than lower end ones. They use better grades of material and are built for more precision for better alignment. They moving bits last longer and are easier to maintain.
For commuting/light touring as opposed to racing, you may want to opt for a frame with more genrous tyre clearance and some threaded eyelets for rack and fender, otherwise you will have to hack around trying to clamp a heavier rack/fender onto some part of your bike never designed for the purpose.