Lets say you have a user-interface with a tree view, and you click on already selected item. Should the click-event be handled normally or ignored (since the item is already selected)? Lets also assume that performing normal click-handling involves lots of lengthy calculations, but in overall takes less than 200 ms.
In other words you can “optimize” away lots of lengthy calculations, and just return right away, or you can force update everything, just to make sure that everything is up to date. As a side effect of second case you can “hide away” some bugs, in cases when some information is not up to date, but user can always rely on that clicking in UI would recalculate everything.