It gives more options to customize layout. Removing the second staircase also removes the need for a hallway all the way through the middle between staircases. Since bedrooms need windows, removing the staircase also opens up space to add 1-2 more bedrooms per floor. Part of the issue with the hallway and 2 stairs is you get a corner unit on each one and everything in between gets just one outside wall. Making a bigger building footprint doesn’t help a lot because the floor area increases more than the wall space(which you need to put windows so your apartment doesn’t feel like a dungeon. Which is why apartments tend to be long and narrow, or sometimes wrapped around a central courtyard.

I read somewhere about North American vs European apartments, particularly the smaller 2-4 floor/3-4 units per floor ones, and the European ones tended to have a smaller footprint, but more wall space and more practically usable space than the North American designs.