If they’re putting down water, then the purpose of that is dust suppression. Which is a thing that gets done, but I’ve also seen tack trucks mistaken for water trucks by passers by
If that’s what you saw, then that wasn’t water being put down… that was AC oil tack, to facilitate the binding of a layer of asphalt about the be put down with the base or binder layer.
These are for suppressing dust, especially when doing dirt work, working on gravel, or even a hard surface in a particularly dusty environment.
If you’re referring to the water system on a drum roller.Water is sprayed on the drum in order to prevent asphalt from binding to that drum.