Air gap in corner, wall?

[jesper---]

70s house lightly insulated, previously only a summer cottage, there is a slight draft by one of the columns at the outer corner, I noticed the wallpaper had come loose so I cut it open.
What is the function of this and how does it work if I want to make a bathroom in this room?

The gap in picture 1 is on the other side of the wall also bordering an exterior wall, but not in a corner. The rest is bordering an outer corner.

 

[Jonatan79]

Shoddy construction

 

[Håbbe01]

A summer cottage from the 70s should probably be taken for what it is. In this case, wallpaper on fiberboard and drafty walls. If you're going to make wet rooms, you need to build/modify the walls according to today's wet room standards.

 

[jesper---]

Summer cottage from the 70s, you probably have to take it for what it is. In this case, wallpaper on fiberboard and leaky walls. If you are going to make a wet room, the walls need to be built/modified according to today's wet room standards

Yes, it needs to be tight, mostly wondering if these slots according to experience serve any function or if it's believed to be safe to fill in and then start the wet room like any other wooden box.

 

[Håbbe01]

No, no function at all.