Why we use error pages in asp.net?

Answer:

Error Pages are used to perform customized actions in case of errors like
403: Forbidden
500: Internal Server Error

In customized actions, we can open specific pages instead of error messages for 403, 500 error codes etc.

This section is written in Web.Config file
<customErrors mode="On">
<error statusCode="400" redirect="error.aspx"/>

</customErrors>

So when error 400 occurs , instead of the error message, error.aspx page of the website should open.