What is client-side routing and how is it used?

Question

I will be glad if someone could answer the following questions

1. How does it work?
2. Why is it necessary?
3. What does it improve?

Answer 1

I was trying to build a Single page application and came to know about client side routing.

By implementing client side routing I was able to achieve the following

1. The front and back buttons in the browser started working for my single page JavaScript application. This was very important while accessing the page from a mobile browser.
2. The user was able to Bookmark/share a URL which was not possible earlier.