Why use private string in c# Bean File

This is not for security. Access modifier does not protect a member from access. With Reflection, anyone can access any members, including private and internal. This is done for the same purpose as all other features of programming languages and systems — to support programming as a human activity, to make programming less tedious and error prone. Access modifiers protect developers from additional mistakes usual when everything is accessible and allowed.
 

—SA

From MSDN (Properties):[^]

A property is a member that provides a flexible mechanism to read, write, or compute the value of a private field. Properties can be used as if they are public data members, but they are actually special methods called accessors. This enables data to be accessed easily and still helps promote the safety and flexibility of methods.

So, if the private variable isn't private, then other classes can get/set the value of that variable. And then the private variable skips the get/set process from the property.