If I'd have to spend my career developing and attending to a single product, a set of libraries would be all I need. I can fill in the blanks and connect them and write and maintain any missing pieces.<p>However I work for customers, many different projects, and I'm more than happy to work with Ruby on Rails, that enforces a common structure to every project, lets me predict where files are and what they do, etc. The advantage is in all the time not spent on learning from scratch every single project. I can start working on the very first day. Even Django, which is mostly anarchy compared to Rails, is better than complete anarchy with every single project using different tools and having a different architecture.<p>Furthermore projects started with Sinatra and Flask and similar tools quickly evolve into something as complex as Rails and Django, with tons of home made fragile code. There is never the time to make them robust, because nobody is working full time on those projects for years. Basically frameworks allow developers to multitask among projects which is great for consultants and also for customers, because they don't have to pay for the extra developers.