Does Android Lolipop 5.1 get encrypted by itself upon first boot?

My doubts are, 1. Would an android device shipped with Lollipop 5.0 get encrypted itself upon first boot? I saw a lot of blogs saying Lolipop does not come with default encryption but in the official link it says that devices get encrypted by default upon first boot — too many confusing blog articles. What is the difference between default encryption and encryption on first boot? 2. Most blogs say we have to manually encrypt the Device but I don't understand — if it's already encrypted upon first boot, then why encrypt again manually? What is the advantage?