Part of it is why carry a physical wallet anymore when your phone (and Apple Watch) can store all of your cards? There are even US States now that let you store your Driver's License in Apple Wallet. (And a version of Digital ID based on a US Passport that works for TSA and sometimes but not always US Customs.) There's an increased ability to leave the house just carrying your phone and not a physical wallet.

In a gym context specifically: a lot of gym wear doesn't have pockets. Being able to leave your phone and physical wallet at home or in a locker and use your watch (which you also use for workout tracking) for every membership card swipe, vending machine electrolyte/protein drink purchase, and gym class ticket can be very convenient.