These are for vehicles with extreme electrical needs... light bars, strobes, spot lights, and so on. A lot of vehicles that have them are bought by road construction companies, fire departments, railroads, etc, with a lot of lighting needs and/or with large inverters for running power tools and equipment. RVers really don't need them for the most part. My field manager job with the railroad involved installing electrical equipment in company vehicles, and many of them had dual alternators.
J & D in Lovely Weatherford, TX
2016 Jayco White Hawk 28DSBH TT
2014 Ram 2500 Crew Cab 4X4 6.7L Cummins