Watcher nodes are computers that periodically (and automatically) run synthetic transactions. If you are using System Center Operations Manager in conjunction with Skype for Business Server, you have the option of configuring "watcher node" computers. In this article Syntax Set-Cs Test User Credential This cmdlet was introduced in Lync Server 2013.
Possible reasons why you would want to clear the Skype for Business / Lync cache include: Skype for Business and Lync cache information on Windows or MAC OS X to read information quickly and efficiently.