Why do we use Twitter bootstrap CSS framework?

Twitter Bootstrap does have some amazing built-in features, namely: A fluid grid layout Responsive design Custom form elements Typography JavaScript interaction Cross-browser compatibility

There are many benefits of using bootstrap as it getting more popularity between web designer & developer community.
bootstrap consist everything in one place as required to all level of website.
It contain support for css pre-processor like LESS & SASS, provide various CSS grid options, Modernizer for Specific CSS targeting specific browser various column, span & margin, padding in organized way.
Another features i love about bootstrap is it having all the media queries based on device so you can have responsive design ready for all devices.
Note : always override the bootstrap & modify according to your need as it is generic CSS framework so you if you override & modify according to your website requirement you can save lot on file size & eliminated the lot of unwanted css & property

Easy to use grid system for responsive design. Excellent amount of documentation and support as well which is helpful during the design process.

It contains HTML and CSS-based design templates for typography

Bootstrap aims to ease frontend development with it's well crafted css styles, taming of browser specific styles, grid system, responsive design layouts and various javascript components.
It incorporates the use of LESS CSS (http://lesscss.org/) to give you the power of using variables and functions (mixins) to build your stylesheets