There seems to be a common pattern where the bottom of a modal window contains an area that looks slightly separated from its content. I'm trying to find out where this habit comes from, and what usability principles are behind it.
Why or when would you use them? Even when there aren't any buttons in there? Where did this come from?
Modal windows (or popups in general) have a lot of quasi-tactile qualities. They seem to break out of the regular flow of an application, and lie on top of it. Does this have something to do with the color of the bar at the bottom of this area? Is there some kind of real-life item we're mimicking?
As a side note; accidentally all examples are social networks, but it happens elsewhere too.
A little more detail about my use-case
I'm currently at a point in a project where we're deciding whether or not to drop that area, as the separation takes a fair amount of space away from the content of the modals. We have a fair few different use-cases, ranging from T&C to contact forms, and some more complex functionality coming as well.
Some of the new functionality is part of the process of entering certain user-generated content, consisting of about 3 or 4 steps, some having buttons at the bottom of the modal, some without.