If you've ever tried to build a website that works well on different viewport sizes (that is, form factors/screen sizes) without the use of a framework to help you along, the motivation for Bootstrap is pretty easy to see—building something like that from scratch is both tedious and difficult. Mobile computing really escalated the need for something like Bootstrap to come along, and it was inevitable that it would. While the same can be said for just about any framework, in that you probably shouldn't spend time reinventing the wheel unless you have an extremely good set of reasons for doing so, it can be argued that (for the vast majority of websites, and even web applications) the frontend has become more important than the backend. It's been a fact for the last several years that the client side is the new black. I'm not suggesting that the backend is not important—nothing could be further from the truth, and i, Integrating Backend Data Services, is completely devoted to building out the backend. However, I am suggesting that when mobile computing came to be, we already had more than enough backend technologies and scores of frameworks to choose from, but were lacking in frontend frameworks.

The final comment I will add to conclude this motivation section is that killing two birds with one stone in the business world can give companies a competitive advantage (that is, speed to market) and/or financial advantage (that is, cost savings)—and so it isn't any different with software development. If you can build something once, in this case, a series of web pages, and use the same client-side code for both mobile and desktop instead of building two sets of everything (or even three sets, considering tablets as well), you should realize savings of both time and money. That is the promise—unfortunately, it is not always fulfilled. However, getting some advantage in these areas is certainly better than getting none at all.