Private fields only used to store values without reading them later is a case of dead store. So changing the value of such field is useless and most probably indicates a serious error in the code.

Noncompliant Code Example

public class Rectangle
{
  private readonly int length;
  private readonly int width;  // width is written but never read

  public Rectangle(int length, int width)
  {
    this.length = length;
    this.width = width;
  }

  public int Surface
  {
    get
    {
      return length * length;
    }
  }
}

Compliant Solution

public class Rectangle
{
  private readonly int length;
  private readonly int width;

  public Rectangle(int length, int width)
  {
    this.length = length;
    this.width= width;
  }

  public int Surface
  {
    get
    {
      return length * width;
    }
  }
}

See