There are no unimportant or unnecessary parts in an assembly. Each component within a product—be it large or small, simple or complex—has been delegated there by an engineer to perform a specific task.
For instance, threaded fasteners are typically used to assemble parts, particularly if the product will need to be repaired or disassembled later. Sometimes, though, one or more types of unthreaded fasteners are needed as well, such as pins, clips, spacers and hooks.