what is network watcher ?

Packet capture tool available in Network watcher.

Variable packet capture in azure network watcher:

Network watcher variable packet capture allows you to create packet capture sessions to  track traffic to and from a virtual machine.

Packet capture helps to diagnose network anomalies both reactively and proactively. other uses include gathering network statistics, gaining information on network intrusions, to debug client-server communications and much more.

Packet capture is a virtual machine extension that is remotely started through Network watcher.

reference:
https://docs.microsoft.com/en-us/azure/network-watcher/network-watcher-packet-capture-overview