The Passcode Is the Weakest Link
You may not think that an iPhone passcode is necessary because of Apple’s strong focus on security and encryption. But all of those security mechanisms go out the window if your iPhone is unlocked. And among pretty much every measure on your device, your passcode is the weakest link.
Even biometric authentication systems offer more security than the standard 4- or 6-digit passcode. If you don’t lock down your iPhone with a strong passcode or password, then you’re pretty much rendering all of the amazing encryption technology on your Apple device virtually useless. (And an alphanumeric passcode is going to be harder for someone looking over your shoulder to remember.)