Back pressure causes some of the restriction in exhaust gas flow, as does the design of the stock manifold itself. Typically, a properly designed header is an arrangement of individual pipes that lead from the exhaust ports into a common passage, which eventually enters a muffler. Obviously, for trucks it needs to be in the lower range where sportsmen need it most. So, bottom line, headers help reduce back pressure, which improves overall combustion efficiency. But there's something else as well.
Headers can lower the underhood temperature, and that can help extend engine life.
I noted that I'd always thought of headers as strictly racing equipment. Jim shook his head at the thought. You should try them out. Which I promptly did. I picked a remote mountain stream so I could put the headers to the test in an altitude climb.