Why we use .net and Why develope .net framework?

In the Old Days when you wrote an application, you started with a blank
sheet of paper and maybe a couple of routine you had written before and
knew worked. As a result it took a long time to get anything done, it all had
to be tested individually, and anyone new coming into the project had a
massive learning curve before they could do anything useful.

Slowly, the situation improved, with first function libraries, then class libraries ,
then a Foundation Class framework. Each iteration moved more and move
routine stuff into a set of objects that were pre‐written and pre‐tested.

The .NET framework is a big, powerful iteration of this: providing classes,
objects, methods and all the supporting bits and bobs which mean that
developers can concentrate on they application specific design, rather than
having to build their own Collections, Buttons, SerialPorts, Sockets,
WebBrowsers, etc., etc..

It speeds up development as a whole, makes it more reliable, and means the
new project members only need to learn the project specific parts.

ASP.Net was developed to make use of OOP﴾Object Oriented Programming﴿
which eases the development process. And the idea of .NET framework is the
ability of gather every module developed using a .net supported language.
﴾for example you can have a VB page and a C# one in the same application
because both is compiled to an IL‐Intermediate language﴿

The other reason .NET doesn't relay on the machine capability, instead it is
running using a framework running on the machine﴾just like Java Run‐time
Environment‐JRE﴿.