Originally Posted by gmercoleza

Overcharging can damage the compressor over time. It may not happen immediately, especially if there is a binary cutout switch installed. Those switches usually shut off the compressor at around 435 PSI, long before any seals would blow (unless there is an existing weakness, but that's another story). But higher pressures will definitely accelerate wear, as can generally be witnessed by the much shorter average lifespan of R12 to R134a converted systems; R134a pressures run in the neighborhood of 100psi more than R12 and really put stress on compressors which were originally designed for R12.

Pressures being out of spec can tell you if there is too much oil in the system, if the TXV is plugged, or if there is some other blockage in the system. It can help diagnose faulty switches and even tell you if the charge is simply low (the most favorable scenario). And yes, condensor blockage would also be detectable with gauges though there would likely be other indicators of that.