well lets see, thats one of those questions thats pretty funny at first, like... why do you use a laptop to datalog and tune your eprom? or even why do you bother upgrading the RAM in your computer? or something. short answer is because I can!
...not to be a dick or anything because you asked. but I can point out some of the more obvious reasons why I use mine and what I have planned for my next system.
I use mine for gathering, displaying, and distributing quick bursts of power to all components in car stereos and engines and accessories. not only that, but aiding in the discharging the battery and buffering of all components in my charging system. It is the first thing to take power and hold it for use and with my large alt wiring aid in displaying big voltage numbers in which my alt can flow tons of clean power and stay cool.
In my next system I will be using a single trunk mount battery. most likely a deep cycle AGM battery. not a crappy spiralcell battery. the plan is to run the charge wire straight from the alt to the trunk with 0 gauge and then back up to the engine bay with 4 gauge to where the stock battery was and have a cap to connect where the original battery cables were. It will serve as the car battery since it can take and make ready power before the system since caps tend to be greedy. I have heard of this system but never seen one but think it will work out great. To me this seems like the ultimate application for automotive use, free up space in the engine bay for turbos and CAI systems and you get a digital voltmeter for free. not to mention you got your power and ground points you lost while relocating your battery. I am pretty excited about getting this project started can you tell //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif