- What exactly does this do?
This will disconnect your MacBook's Wi-Fi from your iPhone's access point. Doing this will not turn off the Wi-Fi on your MacBook.
- Can't I just turn off WiFi on my laptop, or select a different WiFi to disconnect? What problem does this new feature solve?
If you turn off the Wi-Fi on your MacBook, you will certainly be disconnected from the iPhone's access point. Same would happen if you select a different Wi-Fi access point.
With newer versions of iOS and macOS, Apple has streamlined the process of access Internet via mobile data of your iPhone. If your iPhone (logged in with the same Apple ID as your Mac) has an active data plan, your Mac automatically shows the option to connect to mobile hotspot, even without turning it on on your iPhone. Mac also offers to connect to the mobile hotspot on your iPhone in case you are not connected to any access point and do not have Internet connectivity.
Disconnecting from the iPhone access point will stop the Mac from consuming data via your mobile data plan and can save you from consuming data from your mobile data plan (which is generally paid for as consumed).