If you have Apple Watch, you can authenticate services on your computer just by double-tapping the Watch side-button. It also unlocks your nearby iPhone if you have the Watch on.
You can trigger Do Not Disturb across the ecosystem from any of your devices.
Find My lets you keep track of all devices in the ecosystem with a global lost and found mesh network built in.
Apple Pay works seamlessly across iPad, Mac, Apple Watch, and iPhone. My Mac uses my iPhone's FaceID or a confirmation on my Watch to authenticate payments - no setup or weird configuration required.
AirDrop works perfectly for me with all my devices. I can beam documents and data from device to device without thinking about it.
Handoff works across all my devices. I can start an e-mail on my iPad and continue on my Mac instantly.
I can see the charging/battery state of my iPad, iPhone, Apple Watch, and Apple Pencil from any device in the ecosystem, including Mac.
AirPlay works across my iPad, iPhone, HomePods, Apple TV, Mac, and even Apple Watch. AirPlay 2 is incredible and has no delay. Rock solid connection - much better than any other protocol I've used.
If I get a new device, setup and integration with the rest of my devices is instant - no configuration required. This is especially great with AirPods - I don't have to pair them with each device. I just pair with 1 and I'm good to go on the rest.
Apple Store support for everything in my ecosystem.
etc etc etc