The Wi-Fi network as a key

Nobody designs their Wi-Fi password as an access-control system, but that's what it already is. You hand it out once, to people you trust enough to have in your house, and from then on "connected to this network" quietly means "this person is allowed to be here."

That implicit trust is worth reusing on purpose instead of by accident. An app that checks network presence before unlocking a feature isn't asking the user to do anything new — the gate they already passed through is the gate the app is checking. No extra prompt, no extra step, no new thing to remember.

It's not a universal answer. It says nothing about who's physically in the room right now versus who connected once in 2019 and never left the saved-networks list. But for low-stakes, local, "is this basically my house" decisions, it's the cheapest signal available, and it's already been paid for.