Hidden structures in Lego builds

[Denmarks]

This mainly concerns the Christmas Penguin set 40498. If you will look at step 9 you will see what looks like a snowflake. Each edge has a hinge so it can be folded up. Step 10 completely covers it up and it is never opened again. Why is it there? Is that pretty common in sets?

[CastleRail]

There's a heart inside the base of at least one valentine animal set. They're nice hidden steps that provide interest to structural supports and a little reward for the person actually building it