Changing build configurations deletes my dlls (why??)

[spam]

Ok i have a solution which contains a exe and a dll. The exe references
the dll via project references. I build the solution in release mode.
In the release folder I can see the dll and exe built. I change the
build configuration to debug to build a debug version. Note I haven't
built it yet. Just the act of changing the build configuration deletes
the dll from the release folder. In fact the same happens when I change
back to release mode, the debug version is deleted.

Why does this happen?

I'm guessing I'm going to get answers based along the lines of 'it
needs to to know which component the IDE is currently working with etc'
I don't see how this is possible since it should point to the active
one. So what if the other one exists.

And I also assume the delay that I am experiencing (seen with very big
solutions 10+ projects) when I change build configurations it the IDE
quickly building the solutions so the dll exist. Is this correct? Can't
it be like make (from the old days) where you build something and it
stays even if I build something else in another folder???

Thx.