Why use private property setters since member variables can be accessed directly?

In C#, we can do something like:

private string _myField;

public string MyProperty
{
    get { return _myField; }
    private set { _myField = value; }
}

What is the advantage of using a private setter in the property here while we can set _myField inside the class as we want? Why would we want to use the setter of MyProperty?