Originally it was the butler's bedroom.
In the first photo, the light blue paint indicates where there was a restaurant booth in the corner. I'm certain that was added sometime between the first owners and when we purchased the house. The air hockey table is ours though and has since moved to the attic. ;-)
Here's an after picture from the same vantage point.
And the old corner cabinet.
Which inspired the new corner cabinet.
And the view from the corner cabinet back towards the entry to the butler's bedroom. The rightmost door is the entry to the hall; the leftmost door is the butler's closet.
And after from the same vantage point looking back across the room.
The entry doorway has moved to the right side of the corner. The closet now opens into the back hall for seldom used linens and a beefed up radiator to heat the back hall better.