When you set a departure time, you're asking the car to condition the batteries. Preconditioning is for the cabin. Selecting both when hooked to an EVSE means that energy doesn't have to come from the batteries. You can precondition any time, but can only condition the batteries while connected to external power.
Selecting preconditioning when setting a departure time has the cabin conditioning starting maybe 20-minutes before your selected departure time. Conditioning the batteries can start as much as 3.5-hours earlier depending on conditions.
Preconditioning is a people comfort issue. Conditioning the batteries is a range improver. I always do both if I think about it early enough. If it's really nasty out, I'll precondition the cabin before I'm ready to leave whenever I can. Note, for a short stop, when you turn the car off, it gives you the option to keep the HVAC on, which may be less energy than starting from ambient a short time later. It will only run for about 20-minutes or so, more than enough time to fill the tank on the REx, or get a cup of coffee, or many short errands.
