Batteries included systems are a force multiplier. Engineers like writing code more than shipping. Go to any game jam and watch the teams using Unreal or Unity. Their progress dwarfs those that start from zero. While the teams using the full engines are adding game play features the teams starting from scratch are trying to figure out how to load 3D data or debug why their graphics aren't on the screen yet.
It's no different with apps. A good app framework or web app framework gets you to a working state ready to just work on what makes your app different. Starting from scratch means you're still trying to figure out how to get your DB backed up while the other devs are adding features.