The accumulator tank actually holds a bubble of air which cushions
the pressure in the line, keeping down pressure spikes and allowing the pump to run for a while between low pressure "start" and higher pressure "end". The pump raises the pressure in the water, which is close to incompressible, so without the compressible air bubble your plumbing
quickly reaches a high pressure and the pump turns off, then with a tiny change in volume cycles back on and off. Generally short term cycling is not good for the longevity of a pump. You can get the same buffering effect as an accumulator with a stub of pipe or hose at the top of the plumbing
which will hold air.