Bootstrap is an answer to a 10 year old question posited by CSS Zen Garden: Can we make the web beautiful?

Bootstrap is a great default starting point for a website. Its a great tool for internal development where you may have to write an app but don't want to spend days making it look nice. You can do anything you want with it.

If you complain it makes sites look the same, then ask yourself how many of those sites would have just gotten a template from Template Monster a few years back...