An essential covering that covers the main part of a bike, the engine which runs the whole motorcycle. These covers were generally invented for racing purposes, to make sure that oil is not leaking from an engine after the collision.
But soon enough normal travel bikes started having these too when we recognized the benefits of protecting our engine.
These protect the bikes in case the bike falls, so no major portions will come into harm's way, and the rest can be easily taken care of by a little paint job.
Even crash bars are used for a similar purpose, so whenever a bike falls, the crash guard takes the load and does not hit the ground, while not harming the engine.