No clue, I've been around since mid 1.0 (Oct. 2nd, 2016), still never found out what the Treasure thing is for.

Perhaps it's filler space.

Or scrapped content.

Judging by name, it was probably originally meant as a treasure tool. A lot like the ones we craft, but got scrapped with that idea and we have a new system they liked better.

I think I saw some recent China videos that don't even have that slot occupied with anything, so most likely either a placeholder for content that was scrapped, or content they never got around to fully finishing.

If you notice it'd be a weirdly empty spot if it was just 3 squares on the left, and 2 on the right- I've heard of developers filling in spots with placeholders so things look presentable.

I'll make my guess whats that later, i'll eat lunch first xD .. wahahaha