It would show Arenado, Story, and the rest of the team that they want to win now
It’s common knowledge among MLB fans that the Rockies and Nolan Arenado have not been on the best of terms lately. It’s also common knowledge among fans that Arenado wants to win. He’d like for it to be in Colorado (after all, he did sign a contract extension with the Rockies) but the Rockies have not been a good team since he signed the deal. With Arenado’s opt-out clause, he and Trevor Story could be free agents at the end of the 2021 season.
With signing Bauer, it will show both Arenado and Story, as well as the rest of the roster, that the Rockies want to win and win now. That alone could be enough to keep one or both of them with the Rockies beyond 2021.
With the other players, it would be a boost for the clubhouse and their psyche as well. “Hey, our front office signed the reigning NL Cy Young Award winner. We could really do well this season.”