What exactly does "Disconnect from iPhone" do?
What exactly does this command do?
  


This command disconnects your MacBook's Wi-Fi connection from your iPhone's hotspot. Clicking on disconnect will not turn off the Wi-Fi on your MacBook.


  
  Can't I just turn off Wi-Fi on my Mac, or select a different Wi-Fi access point to disconnect? What problem does this new feature solve?
  


If you turn off the Wi-Fi on your MacBook, it will certainly be disconnected from the iPhone's hotspot (or any hotspot/access point for that matter). If you select a different Wi-Fi hotspot/access point, MacBook would still first disconnect from the iPhone's hotspot.

With the recent versions of iOS and macOS, Apple has streamlined the process of accessing Internet on your Mac, via the 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 you turning it on on your iPhone.

Your Mac also offers to connect to the mobile hotspot on your iPhone if you are not connected to any Wi-Fi access point, and do not have Internet connectivity.

Disconnecting from the iPhone's hotspot will stop the Mac from consuming data via your mobile data plan, and can save you from consuming data unnecessarily and unknowingly (which is generally paid for as consumed) by some apps/processes running on your Mac.