What can the NFC on an iPhone actually be used for?

More than people think. The obvious ones are Apple Pay, transit cards and tapping in with hotel or car keys in Wallet. It also reads NFC tags in the background, so you can stick a cheap tag somewhere and set up a Shortcuts automation that runs when you tap it (Shortcuts > Automation > NFC). Mine turns on a focus mode by the bed. It can read a lost AirTag's info if you hold it to the top of the phone, and some ID and passport apps use it to read the chip in the document. Writing tags needs a third-party app, the iPhone won't do that on its own.