Have you ever had your saw cut into the metal of the fence by accident, or perhaps you wanted to burry half the blade into the fence to just skim off the corner of the board? That is why we dress the fence with pieces of wood.
It is easy to do and easy to change -- keeping the metal part of the fence in brand new condition for the life of the saw.
The miter gauge is far too small for most work and has a problem when cutting steep angles of having the wood "drift" into the blade. Dressing the miter gauge is even more necessary than dressing the fence, and has the primary difference that wood slips along the fence but you want it to stick to the miter gauge.