Both good answers.
The hydraulic pumps cannot respond instantly to a change of demand. Therefore, accumulators are fitted to maintain system pressure for the ?second or so before the pump's output is detected and increased.
An accumulator is a cylinder half full of hydraulic fluid under system pressure on one side and a spring of some sort - usually nitrogen gas I think - on the other side of a divider that can move up and down the cylinder. The gas is put into one side of the divider at a particular pressure so that when the hydraulic system is pressurised, fluid filling the cylinder on the other side of the divider forces the divider up the cylinder, until the gas pressure and the fluid pressure are the same. Now you have a small reservoir of fluid at system pressure, and if the system line pressure drops, this fluid is pushed out of the accumulator by the pressure of the gas to maintain the system fluid pressure. When the hydraulic pump output recovers, fluid is pushed back into the cylinder once more.
I hope that makes sense !
Therefore, as has been mentioned, you don't necessarily need accumulators for a hydraulic system to work, but they do make the system work much more smoothly, which is more gentle on the pumps, and also prevents nuisance warnings.
An exception is in the braking system, where an accumulator is often used to provide parking brake pressure, or emergency brake pressure.