Frugal and Energy Efficient Living

You don't want condensate flowing down the inside of the wall, causing the wall to rot, attracting carpenter ants, etc.. Ultimately this can destroy a wall.

If a window air conditioner is mounted correctly in a window the condensate will flow out the back. If water leaks onto the window sill it's obvious.

There's less assurance leaks and overflow will be detected if the air conditioner is poorly mounted in a hole in the wall. You might not see any condensate leaking into the wall.