I'm using it since I own iPhones, and I've also set it up on my iPad Pro. It's easy to unlock my passcode protected device that way. You have to remember your passcode, though, because it's required after restarting and updating the iPhone's software.
What if you lose your device, or even worse: what if someone steals it? The one who finds/steals it can easily use it,change and remove data, apps, settings, all of that within minutes, and you can't do anything against it, except for putting it into lost mode once you notice you don't find it. That's why I'm using a passcode, just to make sure something like that can't happen. I've never lost one of my devices before. It's just because I'd like to be on the safe side.