Server Google Tag Manager Cookie extender tag designed explicitly to extend cookies. There might be multiple situations when you need to use this tag to extend cookies, some most popular scenarios from our experience are:

• A native tag does not extend cookies.
  
  

• Server cookies might be overridden by web tracking.
  
  

• Server and web cookies have different names and standards. A great example here is GA. If you move to server-managed cookies it will create a new FPID cookie. FPID has a different format, name, and technology than GA web cookies. Besides, that FPID can’t be accessed by JavaScript.
  
  

• Cross-site tracking using web-generated cookies.
  
  

• Need to use web-generated cookies for user identification for other platforms.
  
  

Useful link:

    1. Cookie extender tag template. 

    2. How to set up cookie extender tag.