Number two is why I use it. My site has about 2300 pages and regenerating the whole thing when I tweak one part or another of the basic template or change themes is less than two seconds. Extraordinarily fast and encourages experimentation at least for me. Maybe folks with better front end skills wouldn't care but it's great in my experience
1. During development. Having instant builds running whenever you save is great for the development experience
2. For really large sites. Being able to build many thousand pages in about a minute instead of 20 minutes or way more is a huge difference for the authoring experience.