WHAT IS THIS in my closet??

*California home built in 1949.
*Two adjacent bedrooms
*Between them in the hallway is a “box”, it’s the adjacent bedrooms closet.

-I’m redoing my closet, wondering what in Gods name this additional 9.5” wide x 4” deep box is on the left side of my reach in?
My parents don’t have one on their side, only I do. It quite literally stops the reach in from being efficiently used! + I could do so much more if it wasn’t there! However, it MUST serve a purpose right??

My dad thought perhaps a vent to outside? I thought maybe something to do with the attic? Maybe support beams??

Ideas anyone? How do we verify what it is?

If I had extra money I would slice the“box” so each bedroom could have a 24”-27” deep build out / built in closet that runs the course of the wall! (Opposed to the two boxes they’ve given us). Would this be possible??