If you get this as an alternative to jump start cables, then do keep it plugged in as it won't hold peak charge for long. And it does need to be at or close to peak charge to jump start a car. It will power up other 12V accessories for quite a while, so no problems there.
And yes, the variety of options on it (light, LEDs, etc) are nice and much more useful. Definitely better than the spare battery I used to have for such uses. The carry handle itself is a huge benefit.