As much as you or I get out to go fishing, in all likelihood, we spend far fewer days at it than a professional. Since guides are on the water almost daily, they gain an expertise that would be hard to get otherwise. You can benefit from that expertise and most likely get exposure to techniques and information that would take years to gain on your own. Additionally, guides know the areas they fish intimately. They will come prepared with the kinds of flies and other tackle that best represents those of the natural habitat.
Guides also know the waters they work very well.
They see these stretches of water day after day in all kinds of conditions. They will know what kinds of foods the fish are feeding on, as well as where the fish may be in different water conditions such as spring runoff on rivers, or during tidal changes in the salt. Another reason to hire a guide is access. With a guide at the helm, you can get into areas that you can't get to when wading. No trading places at the oars with your best fishin' buddy.