Words are a poor interface for motion. "The steam rises gently" gets you steam, a swaying plant, a drifting camera and a person shifting their weight, because the model has no way to know which of those you meant.
A motion brush makes it explicit: this region moves this way, everything else holds. For product work that is precisely the requirement, since the product must be the still thing while its environment lives.
It also lengthens usable clips. Restricting motion to a small region gives the model less to keep coherent, which pushes drift further out.
Why does everything move when I only wanted one thing to?
Because a text prompt cannot scope motion to a region. A motion brush can, which is why it is the right tool whenever most of the frame must hold still.
Does restricting motion make clips last longer?
In practice yes. Less moving content means less for the model to keep coherent, so visible drift arrives later.