What is the benefit of upgrading my Classic with the Pressure Gauge Kit (PGK)?

By adding the Pressure Gauge Kit (PGK) to your Classic, you are essentially converting it to a Signature: The brewing components and experience will be the same. 

The PGK unlocks the benefits of pressure-profiling, allowing you to monitor, in real-time, the pressure being built within the brew head. You can thereby ensure that you are in the "espresso zone" (6-9 BAR) and crafting genuine espresso. If you're not hitting that target, you'll know your coffee bed needs some adjustment (typically, grinding finer or coarser). 

The PGK also helps to prevent breakages due to the application of excess force, as you will be warned when you are brewing in the "danger zone." If your gauge needle approaches this range, you'll know to carefully terminate your shot and restart with a coarser grind size.