Motorcycle covers are an often overlooked alternative to keeping your motorcycle in a garage or under larger cover. Whilst they are not as ideal due to a lack of airflow, and keeping in condensation (although this is relieved by higher quality breathable and waterproof contemporary materials) they are still great at protecting your motorcycle from the elements.
Even if you do have a garage, then fitting a motorcycle cover will protect from elements inside your garage, such as dust, dirt and insects to a degree. Most garages are not kept at a stable temperature and insulated, so a lightweight breathable cover helps the build up of condensation and therefore mold, which will help protect the finish of the motorcycle.
Outside you will need a more hardcore motorbike cover. It will have to withstand harsher elements, from wind, rain and UV sun-rays. Whilst these issues are not major in small doses, if you are leaving your motorcycle out for a long period (for example, you ride in summer and keep the motorcycle stored outside over winter) then the finish is compromised. UV rays fade paintwork (red is the worst for this). Still water can leave stains (from rain). Wind can take small objects like stones or twigs from trees and scratch the paintwork blowing against the motorcycle.
Motorcycle covers are not perfect, finding the match between breathable and weatherproof is the biggest challenge. You will still need to give your motorcycle a buff up after the bad weather season, but it will be a smaller task than if the cover is not there, and there will be less long term damage to your motorcycle paint and chrome too.
Another big benefit of having a motorcycle cover is for security reasons. Sure, a knife will go through it, even if it is tied down, and the motorcycle is chained to a solid object. But if it's hiding under a sheet, it's a lot less attractive to thieves than them seeing the nice shiny expensive object underneath.
For these reasons a motorcycle cover is an essential item for any enthusiast who cares for their motorcycle, or just wants it to maintain it's value by conserving its finish.