They are useful because they can create life like textures. Another plus for using them is that textures are easy to create! So far I have found only two programs that do this.
Dark Tree 2.5 by Darkling Simulations
Information ~ "DarkTree 2.5 is an advanced procedural shader authoring tool. By combining any of 100+ procedural components into a texture tree, you can interactively create photo-realistic textures, surface shaders, and animated effects."
Other Info ~ Windows Only
Price ~ $419.00 ($20 discount for electronic-only version)
Link
Example:
Genetica 2.5 by Spiral Graphics
Information ~ "Genetica 2.5 is an innovative node-based seamless texture editor for professional 3D artists. With effect maps, more than 500 finished texture presets of all types, automatic seamlessness, and unprecedented flexibility, Genetica will help you revolutionize your textures. "
Price ~
Genetica 2.5 Pro 399 USD for single license.
Genetica 2.5 Standard 129 USD for single license.
Link
Example:
