Art, annotation, memos, hand drawn animation, business'y stuff of that nature
It kinda depends on the stylus though: Some are actual digitizers(pressure sensitivity for use as a paintbrush, some even recognize tilt and rotation for calligraphy) while others are just a rubber nib on a stick(rubber finger...on a stick..no electronics just rubber).
Bundled with every installer in existence with the checkbox checked-on by default.