They help stabilize streambanks, reduce the temperature of the water, and reduce flooding and sedimentation. Riparian buffers aren't strictly agricultural practices. They also can separate the water from roads, railroads, housing developments, factories, shopping centers, parking lots, and other activities. But, chances are that the land adjacent to any body of water is part of a farm or ranch. Nearly 75 percent of the United States is agricultural land - cropland, range, pasture, or woodland.
Buffers do a great job of enhancing fish habitat.
Woody stems slow down flood velocities, and woody roots hold bank sediment in place. Otherwise, you've got an area that's eroding - creating severe bank erosion, wider and shallower channels, and a slew of other hydrological and biological problems. Somewhere downstream that sediment is dropping out. Where the sediment accumulates, you get more flooding with the same amount of water. Any disturbance to the sediment budget of a stream or river creates habitat and property loss, increased flood damage, and other public works expenditures.