Why jack screws
On a large or high-pressure-class flange, the main bolt circle can mean a dozen or more heavy bolts. Backing all of them out just to swap a plate is slow, and repeatedly disturbing a gasket face that doesn't need to move adds unnecessary wear and re-torque risk. Jack-screw holes — usually a small number of additional threaded holes outside the main bolt circle — let a technician turn a few screws to separate the flanges by the few millimetres needed to slide the plate out, leaving the rest of the joint untouched.
Paired with a spectacle blind
Jack screws are most useful combined with a spectacle blind or paddle blind directly downstream of the orifice flange: closing the blind isolates the line, the jack screws crack the flange faces apart, and the plate comes out for inspection or replacement without draining or fully de-pressurising further back in the system.
When to specify it
Worth specifying where the plate is expected to be pulled routinely — proving-run schedules, plates sized for a duty that changes seasonally, or services where fouling means frequent inspection — rather than as a default. On a line where the plate goes in once and stays for years, jack screws add cost and extra machined joints for no real benefit.