The reason being I would to keep the stock alternator and stock battery to run the vehicles electrical system and add a second alternator (not necessarily a HO alt) and battery (I have an Optima D34/78) to run the stereo and lighting. This way there is no strain on the vehicles electrical system and no strain on the stereo. That way everything works the way it should. No loss in vehicle performance and no loss in stereo performance. Plus the cost of most of these HO alternators is ridiculous, it's more cost effective to do it this way, if possible.