It allows you to adjust the amperage that your alternator
puts out. Most alternator regulators taper off the output fairly quickly so you don't get optimal charge rate after the first few minutes. With the Lewco controller you can keep the alternator voltage at the optimum for putting in the most amps to your batteries. IIRC it's 14.2 volts maximum sustained voltage to optimally charge a lead acid battery
. The only problem is you have to keep dialing back the amperage as the batteries charge or the voltage will increase. If it goes too long, you could ruin the battery(ies) or even explode them. The Lewco controller is great for cutting down engine
run time to charge the batteries. That's really important for those who use the engine
alternator to charge the batteries When we were out, about 80% of our engine hours were for battery charging
. The ability to adjust the amperage from the alternator really cut down on the engine run time. The newer after market, expensive multi stage regulators kind of do this for you now if you have one installed.