Because the password is used to unlock the lockscreen. Having a password without having a lockscreen is like having a key to a door that doesn't have a lock.
The lock screen is where you enter any method of unlocking the device (PIN, password, gesture, etc) to access the device. It's built in to the OS. If you don't want to enter a password at the lock screen often, use a fingerprint. It automatically opens the device to the home screen. You will have to enter a password, PIN, etc, when you restart the device or if you manually lock it.
Isn't SWIPE available for a lock screen?
It's not locked. You need a lockscreen with a passcode. The point is if your phone doesn't have a secure lockscreen, anyone who happens to acquire your phone through any means will have access to your passwords and that's insecure.
OK, got it. Evan though SWIPE is listed as a choice for a lock screen,, it's not a lock screen. Go figure.
Look at it this way. If anyone can access your phone, THEY could choose a LOCK screen and lock you out of your own phone.