Pressure limiting valves help ensure system pressures are kept below a desired upper limit and maintain a set pressure in relevant parts of the circuit. Most pneumatic systems are designed to operate within a preset pressure range. Without controlling or limiting these forces, power components (and expensive equipment) could be damaged.
The valves prevent the buildup of excessive pressure or vacuum which can unbalance the system. They are the safeguards which limit maximum pressure in a system by releasing it when it gets too high.