Although it's not a great deal quicker, even when dropping luggage off its quickest when the passenger is already checked in online. No need to worry about seat allocations, printing boarding passes etc. Also the future is self service bagdrop. If the passenger has done the leg work of checking in at home it means less work to do at the kiosk. Imagine you're every day holiday maker that goes away once a year trying to navigate self service check in kiosks, it's only going to slow down the whole process.
Airlines can also gain revenue from online top ups such as seat selections, upgrades, extra baggage, treats on board etc. Computers will ask and upsell every time. Humans won't.
Even if traditional bag drop remains a thing for years to come, the emphasis is on getting as much done before arriving at the Airport as possible.