Powerbank: Works with all phones, Doesn't require you to shut down and take your phone apart, Can be charged with a regular phone charger.
Spare battery: ???
Spare battery: remove empty battery, switch battery, you are back to 100% Powerbank: mess with cables, keep phone plugged until it's charged and wait
If there is a market for spare batteries, there might be a market for external battery chargers as well > swap battery in the morning instead of charging, place empty battery on charger, repeat the next day
I don't need to be able to swap a spare battery like we did 15 years ago.
However when your battery is old, and drains fast, it's really nice to be able to order a new one and easily change it by removing 4 screws.
Not having to perform surgery on your phone, heating it to remove glue, etc.