Why do you use java script ,ajax,jquery to develop websites?

Client Side JavaScript (Ajax, jQuery)

• JavaScript and other JavaScript related frameworks (jquery) and techniques (Ajax) lets you utilize client side resources (CPU and RAM) and frees up server for serving requests.
• JavaScript makes the web more engaging, entertaining and speedy
• As the client machines are becoming faster these days and browsers are becoming smarter and smarter. JavaScript lets you utilize those new enhancements i.e JavaScript based Games, Social Games (that you play on FaceBook).

Server Side JavaScript (NodeJS)

• Async nature of JavaScript is very well utilized by server side JavaScript frameworks.
• They out perform various server side technologies in variety of tasks in terms of Performance and Development Design.
• They lets you develop what we call Non Blocking I/O

For improve UX (user experience) and  developing dynamic web page applications.

Website and Web-application is mainly distributing or presenting the data to users(viewer)

Here, technically manipulating or distributing the data into client(browser) from web-server(Apache).

We feel, while displaying the details(manipulating or distributing) into the browser should be simple, attractive,  flexible to all internet devices and fast loading from webserver,

That's why we are using some client technologies like Javascript, Jquery, Ajax, Json, etc.

It's a way to improve user experience. Imagine browsing on Facebook and for each comment we add, the page reloads completely. This makes the experience boring and distracting. Ajax (Asynchronous Javascript and XML) is employed to interactively load data from the server whilst the page is not navigated away.

I would say Ajax is quite different technology than the rest- because with ajax you are retrieving and sending data to server without changing any display of existing page - means you can keep your focus or make changes on the list box and instantly you can focus on other options too- whereas if you want to achieve this functionality in asp.net you need to work on post back event that takes time on front end area.

For better user experience and speed of the site. Ajax mainly faster the website by loading the content of the site without refreshing.