I am playing Classic WoW (WotLK and SoD) and a few versions back the icons for "Open Bank" was moved and the original position got replaced by "Focus Trade". Now I fall for this change quire regularly and this new option hides the entire bag.

Can someone shine a light on me why the position changing was done in the first place and why in gods name an option which hides the bag was introduced at the same place?

How is that relevant for Classic though @Jaliborc ? In reality it simply hides the bag for these WoW flavors.

I suggest to show the bank for these flavors, like it was before, instead of having some sort of "dead button".

Focus Trade/Normal is a shortcut to toggle between normal/reagent bags.