Because we’re humans and ride these things called bicycles, we are continually trying to ride faster and more efficiently -trying to optimise what we do! The biggest hurdle is aerodynamic efficiency - the drag on a bicycle accounts for 80-90%, and the bike accounts for 20% of that. The more we can reduce drag, the better.
If all else is equal, a rider on an aero frame will go further and easier then on a non-aero frame, due to the fact that you can put less power in to achieve the same speed!
And yes, if you’re a slow rider, or a fast rider, you’ll still benefit from the aero effect!
How about the fact that aero bikes are heavier? Doesn’t that make it harder to climb? Well, when looked as a system, (bike and rider) the difference between a light weight frame and an aero frame is negligible, and the aero advantage can make up the difference!
So, buy an aero bike. You’ll be faster!