It's not useful to watch, almost by definition. If you're an insider obviously you want to sit on the information as long as possible and make a big bet just before it will be revealed. For example: the account in TFA whose bet was 71 minutes before the first news article about the attacks.

So how useful is it really to see a big bet and know that probably the thing in question will happen in the next hour?

The only kind of use case I can see for these markets is what another commenter mentioned, as a kind of strange insurance by betting against what you hope will happen. But even then, the finicky rules and untrustworthiness of the Polymarket admins make them much less reliable than a traditional insurance policy...