A luggage net is one very convenient way to have extra luggage capacity at your disposal at any time and place.
Requiring only a very small storing place, it still offers a lot of unique uses. For example, there is hardly a better way to carry an extra helmet, jacket or pair of gloves, without installing a case. Actually, it can hold just about anything you can't safely squeeze under an rubber band, and then some.
Given the versatility of a motorcycle, and taking the wind, elements, and smooth surfaces into account, a luggage net is just about the most versatile way to carry luggage in a simple and safe way.
It should come standard with every bike.