It saves you from this scenario:
Your at some public place where is optional to publicly bump your system and a few hours later you decide to cruise on out and you turn the key and click click click..no power.. Now if you have the Voltmeter you have a constant indication of the power of the battery and when you need to turn your car on for a quick charge so as not to end up in that situation mentioned above...
Aaaaand im spent
been there several times... got the volt meter and now i dont have a problem
Besides this, I often sit in the car for many hours and do written paperwork at the beaches of San Diego (California) when I need to take a break and put some planning down onto paper (SD is about 100 miles south of where I live... so its more of a mini-vacation)
The volt meter is also great to see if you are getting spikes or if your car is near dying just as you accelerate from a stop - it helps to monitor your alternator/battery system when you put allot of demands on your car's electrical system