Say you're on the bus stop and want to check upcoming buses (real stuff in the place where I live).
The bus company could either slap a QR code, or a "bit.ly/bus-stop-1234" URL. And someone could paste over it a "evil.com/bus-stop-1234".
Hint: use Firefox Focus as your default handler for URLs on mobile phone. It clears all history and cookies after each usage, which is perfect for opening unknown URLs.