I think you have to minimise the information you give your customers. You want to sell the damnned bike, not give a seminar on biomechanics. Also do a show and tell rather than just tell.
I would say that gears are their to match your power which is constant to the terrain and conditions, which vary. People dont really have an accelerator pedal, once you are riding you cant produce any more power.
Low gears enable you to ride slowly over hills , rough roads and with heavier loads (put the bike onto a low gear at this point). High gears enable you to travel faster downhill and with a tailwind (shift the bike into a high gear). Medium gears are good for most riding on flatter terrain (stick it in the middle ring and a med rear sprocket). Show them the position of the gear shifter and the chain at each shift.

If you have a triple chainwheel, use the small ring for difficult terrain, the big ring for very easy terrain and the middle ring for most ordinary riding.

If pedalling becomes difficult, change down.
If pedalling becomes too easy, change up.
If you notice that your chain is crossing (show them), change the front ring and find the duplicate gear