When one begins to pay more attention to small pressure changes in how handling and rider comfort are concerned, this is where a small light gauge really shines.
Going from too harsh pressure to letting some out but not actually knowing the pressures is the key issue here.
I ride bikes with different tires with different loads on very different surfaces and speeds. The operating pressures can range from 25 to 100 psi , and the very apparent riding changes that come from a couple or 5 psi can be very much felt and appreciated.
So if you are a rider who wants to improve handling or your comfort, and experiment with different pressures, and are atune to small changes--or even learn to be more sensitive to pressure changes, then a hand held is a very effective tool.
In all disciplines, small differences are the key to important improvements, and with tire pressures, it's fun to play with pressures and feel how they change how you can corner, ride faster over given surfaces, have more confidence in your front end which invariably leads to you being faster.
I guess for me I knew this from racing motorcycles a bit 35 years ago, and over the last while have become more aware of my bicycle pressures and the advantages of X vs Y psi and how changes make me a more comfortable, faster rider, while being able to confidently push harder in corners on various surfaces and conditions.