The use of appearances can make a complex assembly model easier to understand. By default, assembly components display within the assembly using the "As Material" appearance. A part that was defined as "aluminum" uses the "aluminum" appearance. A subassembly with an aluminum part and a brass part displays each one in its respective material appearance. 
It can be useful to display an assembly component in an appearance other than the material appearance. For example, all components purchased from a single vendor or designed by a particular designer can be displayed with one appearance. Displaying an entire subassembly in one appearance (instead of the unique material appearances) can simplify the display. Using a transparent appearance (such as the predefined "glass") can be useful if a component obscures others behind it.