I am new in MVC 3. What is the reason to use two web.config files?

Wha
This is an example of web.config file inheritance. From MSDN
You can distribute ASP.NET configuration files throughout your application directories to configure ASP.NET applications in an inheritance hierarchy. This structure allows you to achieve the level of configuration detail that your applications require at the appropriate directory levels without affecting configuration settings at higher directory levels.
Specifically, for MVC projects, the web.config in the View subdirectory is used to tailor the .cshtml / .aspx files. You can use web.config files in subfolders to extend, override, and remove settings inherited from the app's own root, and further up the hierarchy, e.g. up to machine.config
Common configurations in the /Views/web.config include:
404 response is configured for such direct requests, e.g.
using. You can add namespaces for your common custom assemblies here (e.g. custom html helper extensions) e.g.
...
RequestValidation filter for MVC. The comment added in the config explains this best: