why is .cs file used in C#

we will write logic code in cs file only...
the extension is .cs..

Basically you will find two types of cs files in a asp.net application. One is *.aspx.cs and another is *.cs

The first one works as code behind file for a *.aspx file. For instance, the most common file you find in any application is default.aspx & default.aspx.cs

Default.aspx file contains all the UI (HTML) which you want to show the user and Default.aspx.cs file contains code to manipulate the content in the aspx file. You can have a Label control in aspx file to display logged in user name & you will write code in aspx.cs file to set the logged in user name in that Label control.

*.cs files are mainly class files created by developers to write extra logic for the functionality. For example, you can create a helper.cs class and write code for return the name of the logged in user. You can then call this function from the default.aspx.cs file to display the logged in user name. The benefit of creating the class file in this case is that it can be reused on any page, i.e. you can display the logged in user name in any aspx page by calling this class function from the aspx.cs file.

Generally adding an aspx file to the project adds an aspx.cs file automatically. For *.cs file, you will need to first select additem and then select class file from the list of file types.