An avid motorcycle tourist will tell you that two major necessities for riding are the handling of the bike and rider comfort. If you are just getting into motorcycling, don't be misled into thinking that the most expensive bike will automatically provide these qualities. In many situations, changing the factory rear tire on the motorcycle to a wider one will give the rider better grip and handling, especially on bikes that are supported by rigid frames. This is accomplished with an increase in surface area, which in turn absorbs more shock and road vibrations.
When cruising or touring on a motorcycle, it is important to have a machine that handles well. A wide rear tire helps keep the back wheel aligned with the front wheel throughout the turns. A wide tire will lessen the chances of the back wheel breaking loose, causing a wreck. Also, a wider rear tire will hold a better line when riding in the rain, eliminating the "skipping" of a skinny tire. A wider tire may also help transfer the power of larger motorcycle engines, lending increased acceleration and breaking as well as greater stability at higher speeds.
Having a comfortable ride is always nice and essential when taking long-distance tours. This is another advantage of replacing a factory tire with a wider one. More surface area on the ground means that there is more room for absorption of little bumps and gravel in the road. Additionally, a wider tire marginally reduces the stress of the rider in holding the bike during minimal or zero motion.
Appearance is probably one of the biggest reasons people who own custom bikes have wide back tires; a wide rear tire flows with the style of the bike that employs a "beastie" appearance. This is often called the "chopper" look, which many enthusiasts seek.
If you are considering switching your rear motorcycle tire to one that is wider, make sure you also change the rim to accommodate the size of the tire. Tires that are wider than the rim reduce the amount of contact patch, resulting in a rounded bead of the tread. Trying to compensate a wider tire with a regular rim by reducing the air pressure will gain traction and lessen stability. The correct rim width for tires is essential to handling, stability and safety.