Effectively, with the range of it's capacity and the bladder set-up, provide for you to take small amounts of water out without triggering the pump to engage from it's pressure switch.

So, if you have a 1 gallon accumulator (or so), you could take-out about (say) 1/4-1/2 gallon with before you heard the pump go on. There is no free ride, when the pump goes on, it will pressurize the system, including the accumulator.

It keeps the pump from short cycling and maintains a more even flow to the fixtures. If you use the water in short bursts, you would tend not to get just the pump pushing the water through the lines.

The newer variable pressure pumps substitute a more sophisticated controller to accomplish the same thing.

I haven't used a varible pump -- but the accumlator tanks have worked well for well over 40 years on boats. The pump and controllers are less expensive and simplier. The newer pumps adjust the pressure by virtue of modulating the rpm and current draw. (Like a variable frequency drive on a larger piece of mechanical or plumbing piece of equipment).

The first cost of both is probably a bit lower for the variable drive. I think the quality of the "experience" is better with an accumlator. Some would say that the accumulator's are quaint ;^))). I'm almost feeling "British" about this subject.