I'm reading up and it seems data watchers are thread safe (auto lock/unlock) and also automatically synchronize between the client and the server.

 

I'm wondering when I should use them and when I should use regular class fields? Also, there is a health field in Entity but in the Wolf code it also tracks this.getHealth() on entity init?

 

Could someone explain exactly what are data watches are used for?

Data watchers are used when you need to send server information to the client or client info to the server. In most cases, this is going to be a variable determined server-side that you want to use for rendering.

In your example with the wolf, the health is only calculated on the server side, but the wolf's tail needs to render at an angle based on its health.

In some cases, the variable will be calculated the same on both the client and server sides or the variable is only needed on the side it is calculated by, so data watcher is not needed.