Actually, the bleed hole does allow an equalization of pressure between the panes, serving to put the pressure on the outer pane, reserving the middle pane as a back up in the extremely unlikely event that the outer pane would fail , but also serves to release the moisture from the gap reducing most fog and frost.
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Thanks for the developed answer) you are right)
The best answer for sure - I spent a fair bit of time travelling (in my mode as Cabin Attendant LOL) and always thought one of the main reasons was the mist one. Thanks.