As of bug 8075 a popup is shown in Web Admin when adding/editing items. Opening the popup causes the page below to scroll back to the top. This ruins the illusion of a proper popup and can be problematic in pages where you could have a very long list.
One such example is the Sessions-page, you could have hundreds, or even thousands of sessions. Clicking one of the sessions would bring up its details, but it would also scroll the underlying page to the top. And after closing the session details, you will have lost your position in the large list.
Note that, technically, the popup is actually a different page. Opening/closing the popup involves navigating to a different url.
Closing the pop-up is more important than opening it. When opening, the issue is only cosmetic, as you cannot interact with the background page. When closing, though, you probably want to continue at the position you were before opening the pop-up.