Do I need a lock screen?

Yes, if you use Quick Lock or a Device Lock schedule. Both lock your whole phone, and your lock screen keeps you out while the lock runs.

Use a PIN, pattern, password, fingerprint, or face unlock. A swipe-to-open screen has no security, so a device lock cannot keep you out.

Blocking apps is different. It only blocks the apps you choose, and it doesn't need a lock screen.