The iPhone could be a Thread border router, but it probably shouldn’t be
The iPhone could control Thread smart home devices faster
Thread could replace Bluetooth as a way to connect peripherals like the Apple Watch to your iPhone
Thread would be fine for most smartwatch uses but not for audio streaming
Thread might just be a bonus radio
The only Matter devices I’ve seen so far don’t support Thread, but work over WiFi.
Pretty much everything produced Eve (besides their cameras) and Nanoleaf is Thread. They were once only available for the Apple ecosystem but since the Matter standards were released they have been venturing out.
Wifi is a bit of an odd choice IMO. It’s not a great smart home device protocol.
If you want to support a lot of devices in a dense apartment building with wifi you pretty much want everybody to have MU-MIMO capable access points and IoT gear. Otherwise you’re hogging spectrum.
(It’s not like other protocols aren’t also affected, but at least they don’t compete with people’s internet traffic)