They know the regulations, so you won't accidentally end up on the wrong side of the law. Plus most guide services supply transportation to and from the fishing area, a boat, meals and drinks, and sometimes even equipment. If equipment isn't available to loan, it's generally available for sale at the outfitter's shop. Package deals that combine lodging or breaks for additional people are common. Boating with an outfitter offers many advantages.
Outfitters also supply the food, transportation and local expertise.
You can generally count on boats and other gear to be reliable and the guides to be trained and familiar with the river. Many companies pride themselves on their guides' knowledge of local history, flora and fauna. Ratings can change with water flows. The difficulty of a section is also tied to the craft you choose and your proficiency with that craft, so the information provided by this scale should be used with caution. A class IV, V or VI river will be mostly or completely turbulent.