I didn't really get it until I Airdropped a shipping label PDF to the guy at the UPS store to print from his phone, which was already set up for the label printer.
My mum can use airdrop. She sends us things. She doesn't need to install anything or configure things. She just uses the first-party UI.
It's the same with Airpods. Bluetooth headphones? Yeah, you can buy some for $20, who needs Apple ones? Apparently everyone. Turns out providing great noise cancellation and bluetooth that just works means the product is appealing to general consumers.