The original purpose of the fitment of an OPV or PLV(Pressure Limiting Valve) to espresso machines with Vibe Pumps, was not to control the pressure at which espresso shots are poured but to protect the pump and the high pressure circuit from being subjected to excessive pressures.
Excessive pressure will cause a Vibe Pump to stall and eventually cause damage to its internals, plus the possibility of burning out the Pumps Armature Coil. Machines that use an Eccentric Vane Pump, or a Rotary Pump (as it is more popularly known within the coffee industry) dont suffer from the same problems as the Pump Housing is designed to include a Bypass Valve that allows liquid from the High Pressure side of the Pump to recirculate back to the Low Pressure side when ever the Bypass Valve pressure setting is exceeded. These are essentially self controlling.
As you can probably appreciate though, if this situation was allowed to continue unabated for an extended time, the temperature of the water that is being continually recirculated like this will slowly but surely continue to rise (due to friction losses, etc) as high as boiling point (and even beyond) until the pump seals fail and the pressure can be released. So, even in the case of this pump design, there are limitations.
Anyway, hopefully Ive explained the gist of why the OPV or PLV exists.....

Cheers,
Mal.

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