I actually had this argument with someone once. His contention was that he could do just as well with his high chart. I agreed, but stipulated that you can do the same thing in half the time (even if you're talking 2 minutes vs 5) and get a predicted fuel consumption log. Work smarter, not harder.
You can request optimum jet route, or input your own legs based on preferential routing or operational mission requirement (great for IFR-VFR-IFR).
The system will account for pretty much every USN airframe, any model, any engine, and accounts for icing bias, climb and descent, and drag count.
Disclaimer: I had my own doubts about using a computer to do my b1tch work, but in 6 years, I've never been let down by OPARS. I had a flight from St Louis to Oahu, and my FE was freaking out because winds enroute were showing we would flameout over the Pacific. OPARS predicted a steady tailwind once we made the turn southwest. Dead on, we hit the FAF with 6K over our on-top min. Learn it, live it, love it. Plus it's easy.