The role of a lens is pretty clear for a nice sharp image, but how much does the camera body matter?
What are the extra features offered by a higher end model? How much difference do they make?
It is often the case that high-end cameras even use the same sensors as entry-level ones. Still, there are more advanced sensors used top-of-the-line models. These sensor tend to either push high-resolution, like the Nikon D800/D800E or high-speed and ultra-sensitivity, like the Nikon D4S. This is a compromise even a professional photographer must decide on, yet has obvious advantages.
The more general difference though is that mid-to-high-end cameras more direct controls. These impact the speed at which a photographer can operate the camera and set the settings they want quickly. The faster, they can get the camera to do what they want, the more chances of capturing a moment.
The viewfinder of high-end cameras is better which help compose the scene perfectly and inspect what is happening more clearly. This is because professional DSLRs all have 100% coverage viewfinders, while a few mid-range models do. Magnificiation is higher too on such cameras.
A tougher more durable body is an important factor in reliability. Most high-end models are weather-sealed and some are freezeproof too, which is a feature rarely seen among entry-level cameras. This lets you use the camera in rain, snow, sand-storms or other incelement weather which makes it possible to capture images in more conditions. Note that the use of such bodies, require compatible weather-sealed lenses as well.