The update fixes that nasty Wi-Fi bug:
watchOS 4.0.1 fixes issues that in rare cases were causing Apple Watch to join unauthenticated (captive) Wi-Fi networks, such as those found in public places like coffee shops and hotels, which direct the user to a web page before the network can be accessed.
I caught my watch trying to connect to one of these networks just the other day. I hope this takes care of things.