Just like in the real world, changing your paper type can drastically alter the appearance of your work.

Clicking on the center icon at the top of the tool panel will toggle the state of paper texture.  This is the basic setting for paper, however there are additional options...

The Paper panel has a couple of controls that will let you fine tune your paper texture.

The Intensity slider lets you change how much the paper effects your paint.  A high value will give you a very rough texture, and a low value will give you a very light texture.

The Bumpiness checkbox enables basic paper textures.  It gives your paint a texture with raised and recessed areas that has a 3-dimensional quality.

The Drybrush checkbox enables an effect that mimics the way paint settles into the texture of the paper.  It is greatly effected by the paper intensity, as well as the brushes opacity.

The Invert checkbox causes forces a negative version of the paper texture.

The Paper scale slider changes the size of the paper texture.

By altering the various parameters, you can build up interesting patterns.

Some artists have used paper textures very effectively...

You do not have to rely on the built-in paper textures either.  You can load a new paper texture by clicking the Load button.  You can also use your current custom brush as a paper texture by clicking the Use brush button.

Another tool that makes loading paper textures simple, is the Patterns panel.  The Use as paper button will cause the patterns to load in as new paper textures, giving you a 'one-click' solution.