Hydraulic oil can do an incredible amount of work very efficiently. It can also cause extreme damage to pumps, motors, hose, fittings - the weakest link - when there is no safety relief valve.
Every hydraulic system should have at least one, if not two relief valves. One main relief valve and one backup relief valve. The main relief will ensure the system does not work above a certain pressure. The backup relief valve will kick-in should the main relief fail and is usually set 200-300 psi higher than the main relief.