mvc vs asp.net why use mvc

ASP.NET is the actual web development framework provided by .NET framework. MVC, Web Pages (to be consumed by ASP.NET MVC since ASP.NET 5 a few updates later) and Web Forms (No longer included since ASP.NET 5) are just the derivatives of this framework. What you did miss was that MVC is also a three-layer architecture in ASP.NET, it converts your application's code into three layers as,

1. Model - model is the data access layer.
2. View - View is the user-interface of your application.
3. Controller - The main back-end code for your application; business logic.

You can add the code for your data-access-layer in Model, the code for your user-interface in the Views and the code for your business-logic in the Controller section. There won't be any difference that you will find out in either frameworks; but you will get a lot of more features and functionality in ASP.NET's MVC framework, such as URL Routing, Entity Framework; which can work off-MVC too, and many more such other features.

If you're a newbie, then you can learn ASP.NET MVC from their official documentation[^], or you can read my article on ASP.NET MVC to understand the concept underlying in ASP.NET MVC framework.

Understanding ASP.NET MVC using real world example, for beginners and intermediate[^]

Check this out: Learn MVC (Model View Controller) step by step in 7 days – Day 1[^]

There are pros and cons to both approaches, however from what I've read WebForms (the proper term for what you call "asp.net", MVC is still asp.net) is being phased out in the new releases of the framework and dev tools. If you're still using asp.net 2.0 then there are already many changes and advancements between what you're using and webforms in .net 4+. I'd definitely recommend you move on from 2.0, but if you want to move on to mvc rather than just update your webforms knowledge then that would be no bad thing, as things tend to be moving in that direction anyway.

You can google "webforms vs mvc" and find a lot of articles that cover some pros and cons, but this is one instance where I would probably recommend against doing research on the net, as almost all articles I have read on this topic are incredibly biased and lacking in any real technical depth. For example any "mvc vs webforms" article that mentions "viewstate" can immediately be ignored :)