If you needed a tool that would best describe an HVAC technician or his company, it would be his manifold and gauges. Other than hand tools, that’s the one most technicians carry. So, it would be unusual to think of a tech going on a “preventive maintenance” or “tune-up” call without connecting this device.
Service agreements and tune-ups or “preventive maintenance,” as is required by most A/C equipment manufacturers to fulfill warranty obligations, too often is poorly done by servicing dealers. Frequently, more harm is done than good.
I recently read an article in a local HVACR magazine that somewhat addressed this problem, which I guess we all should have foreseen coming because R22 is getting very scarce and its price is skyrocketing.