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 by Eve (besides their cameras) and Nanoleaf is connected via Thread. They were once only available for the Apple ecosystem but since the Matter standards were released they have been venturing out. Thread is relatively new but it’s use in new devices will only grow. Thread and Zigbee use the same wireless protocol (IEEE 802.15.4) but differs from Zigbee products for it doesn’t require individual homehubs for each separate company. Thread products also benefit from power consumption that is similar to that of Bluetooth but has a range on par with Wi-Fi. Thread device also act as gateways or a tunnels for passing along commands to other Thread devices on the Thread network so where each additional Wi-Fi based device further burdens the network each additional Thread device only strengthens it. Needless to say I’m a fan of Thread.
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)