Bootstrap is an open source project created by Twitter. The goal is to make website responsive. Responsive meaning the layout of the website will change if the screen size changes.
Bootstrap is a CSS alongside with JavaScript. Aka bootstrap.css and bootstrap.js
The bootstrap css contains predefined classes that will make a website responsive. For example. If screen width around 450px wide, show a webpage that is 400px.
To use bootstrap, you would have to link it to your webpage that is going to use those classes with the link tag.
Does a website need to use bootstrap? Straight answer is no. You can build the whole website yourself provided that you know how css works.
Straight answer, I don’t know. LOL
I can imagine using bootstrap would make your life a whole lot easier. But I am not so sure that all companies would use it to build websites for client.