I'll try to expand the topic.. after we all come here for this.

In simple language:

watch - like taking a quick peek at your notebook. You open, you look, you close. For example: "What's ur favourite color?", you say Blue. But if you change it to Red, i'll have to open the notebook again. ( snapshop )

useWatch - like a helper that all he does is looking at the values, once the values change, the helper shouts the value.

In our language:

watch - Pull mechanism.

• Snapshot based value retrieval.
• synchronous snapshot of the current form state ( or specific field ).
• On Demand ( returns the current form / field value at the exact time of execution )
• No-rerendfers ( accessing the state without affecting the components lifecycle )

useWatch - Push mechanism.

• A subscription to form fields
• Re-renders ( components remains in sync with the form state - suitable for dynamic ui updates )
• Designed to be used inside functional components to create responsive and reactive elements
• Proxy based tracking ( uses JS proxies to optimize reactivity without performance overhead )

Bottom line...

When performance is critical - watch to avoid rerenders.

For complex ( form in form, dynamic fields etc... ) - useWatch to maintain synced state

react-hook-forms is actually a very good choice! But in order to really master it ( or at least answer the 'how' or 'why it workes so fast' ) you should check the JS proxies and Controlled vs UnControlled components in react, refs, this is the root of RHF.