Bootstrap has evolved over time, along with the web, and is now a mature framework.
By using it from CDN, we can lift the burden of hosting it on our servers. There are thousands of other websites using it over CDN, so we can rely on the fact that users have probably visited some other websites before using ours. Our website can use the pre-cached bootstrap files from disk, so our website can be drawn to the users screens quicker.
Bootstrap has built-in responsiveness, so every out-of-the-box class helps us develop a page that fits on the screen, no matter the size.
