Fuel Pressure Gauges are used to ensure that your fuel's pressure levels are optimal, which in this case means not too low.
More on Fuel Pressure Gauges - Diesel Trucks & More
When a fuel pressure gauge says that pressure is low, damage to fuel injectors and pumps can occur, resulting in a loss of horsepower. Too much pressure is also bad for the engine, forcing too much fuel through the injectors and resulting in a "rich" running engine. The symptoms of this can be very serious, ranging from fouled spark plugs, too slow or too fast idle, to even black acrid smoke which emanates from the tailpipe! Sometimes, low fuel pressure means that your fuel filter is clogged. Listen to the fuel pump, see if it's running, and take it in to a shop if it doesn't sound like it is. It's very important to monitor your fuel pressure, and fuel pressure gauges are the best way to ensure that you know what is going on with your system before it becomes a catastrophe.