With the inclusion of Victor Caratini in the Yu Darvish trade to the Padres, the Cubs will have to acquire a backup catcher, since Jed Hoyer has said Miguel Amaya won’t start the season in the big leagues.
Let’s first assume that the Cubs aren’t going to trade Willson Contreras.
Jason Castro could be a useful backup to Contreras, and then perhaps serve as a mentor to Amaya in 2022 if Contreras is dealt, or let go after one year if the Cubs wanted to go with a Contreras/Amaya combo.
This article by Patrick Mooney in The Athletic, while primarily talking about the possibility of trading Contreras, noted that the Cubs were in fact interested in Castro:
In terms of next moves, the Cubs have shown interest in Jason Castro, sources said, viewing the veteran catcher as a good defender and a possible left-handed-hitting complement to Contreras.