The ability to use it when BT fails and the ability to use it when the charge runs down.
Over the last month, I've had to instances where this was extremely useful. One where the BT on our new iMac running Mojave "freaked" and simply turning the keyboard off and on didn't reestablish a connection even though the charge level on the keyboard was around 85%. Connecting the cable healed the issue.
The second instance was a case where some SFB office person turned off BT, presumably via the menubar icon (now disabled, BTW) and they seem to have either forced a shutdown or left it up overnight. While I had to borrow a wired mouse from another machine in the gallery, it was convienient to simply connect the keyboard with a cable and not have to rearrange the whole workstation to connect another keyboard.