Just for clarification, as used in the i3, preconditioning is for the CABIN. Setting a departure time is what conditions the BATTERIES. In that operation, you have the option to also precondition the cabin. Doing both means your battery is still full, at the optimum temperature, and the cabin is comfortable when you leave. This should decrease the load on the battery assuming you want to warm or cool the cabin as you drive, at least that initial conditioning period. Depending on the ambient temperature, warming the battery may or may not be beneficial, but then, it won't waste energy either, since the car just won't do it if not required. During driving, the use of power from the battery pack warms it, sometimes, enough that it takes active cooling to keep it in a safe range.
You cannot precondition the battery pack unless you are plugged into an EVSE, but you can precondition the cabin any time you wish, regardless of that. Be aware that, depending on temperature, it can take upwards of 3+ hours to condition the battery pack, while the cabin preconditioning will only run for 30-minutes. That's enough time in the winter to make it easy to remove any remaining snow or ice that may have accumulated on the vehicle, and take all of the chill out of the interior. Nice to sit on a seat that is warm and not have it suck heat out of you even if the air temp is comfortable.
2011 535i x-drive GT, 2014 i3 BEV