Gauges are a way to quickly and accuratley monitor the operating conditions of the engine and transmission. The factory "gauges" are slow to react, give vauge readings, and are not accurate. I like to know whether I am getting 60 psi of oil pressure or 5 psi. With the factory gauge you would see "normal" pressure across all pressures. The same goes for transmission temp, coolant temp, egt, and boost. I watch the trans temp gauge on my truck show a completely warmed trans, while the ISSPRO gauge does not even show 100*. That is not accurate. I see four gauges as mandatory for a diesel. They are: pyrometer(for keeping an eye on egt's while towing), oil psi(for showing the correct pressure at all times), coolant temp(to quickly and accuratley monitor coolant), and one of the MOST IMPORTANT, TRANS TEMP(allows you to accuratley monitor transmission fluid temp to avoid overheating, even on a stock unloaded truck). I keep a trans temp gauge on everything I own with an automatic trans. That is why I have certain gauges.