One big addition the Mariners made last year was Teoscar Hernandez, who posted a pretty average 105 wRC+. That was down from his production a year before when he posted a 130 wRC+ season, but that didn't stop the Twins from entering the chat during the trade deadline as a potential team that could land Hernandez and use him as part of a postseason run.
Seattle hung onto him and tried to pull off something similar, although it ended up on the outside looking in. Hernandez is a free agent this winter and likely to sign somewhere else, which creates an immediate need for a starting right fielder.
Cade Marlowe was called up this past season to help with depth, but if they want some instant impact then trading for Kepler is a perfect fit. He’s an upgrade over Hernandez, bringing a 124 wRC+ with him after the season he put together in Minnesota, and could become the everyday right fielder as Marlowe and others add depth behind him.
Kepler can move around the outfield too, if that’s something the Mariners wanted to experiment with in terms of lineup possibilities. On any given day, though, Seattle would last Julio Rodriguez, Jarred Kelenic, and Kepler in its outfield, plus the ability to have Kep hit in the DH spot.