Technicians need to verify superheat to ensure compressors are operating well. (See related article by John Tomczyk, "Understanding Superheat," in this issue.) Subcooling is also important to check, especially for newer refrigerants like R-410A, to verify that the condenser is performing as designed. Both superheat and subcooling usually require several measurements, PT tables, and math by the technician. There is potential for error.
At the same time, many faults may be difficult to detect with traditional tools. A thermostatic expansion valve that is hunting slowly and fluctuating over several psi is very difficult to isolate using a needle or purely digital pressure gauge.