This framework allows you to create and maintain long-term viable applications that make use of declarative formats, clearly defined API contracts and processes.
The eponymous twelve factors include aspects like codebase, dependency management, configuration and service management. These factors serve as recommendations and best practices as to how a web app should be constructed in order to be both maintainable and sustainable in the long run. You can use these factors as a checklist for your own applications, which will give you a clearer picture of how these applications fare in terms of quality and of what you could still improve upon. In addition to this checkbox-like character each of those aspects is explained on the website in far more detail.
Apart from its usefulness in terms of actually developing web apps I find the Twelve Factor framework to be conducive to teaching others about technical frameworks like Spring Boot or Angular, too. When applying particular tools and techniques it’s always good to regularly take a step back and consider why we’re using a certain technique or pattern and how it’ll help improve our design. The Twelve-Factor App serves as an expedient frame of reference for learning about web app development as a means and not just an end to itself.