I can imagine some
middle-level manager at Stack Exchange telling the developers, "Hey guys, let's put a search bar on top and label it as a search bar, but surely that will confuse our users, so let's put a search icon to the right ... but that's no fun, let's make that icon visible when the user clicks on it, because you know, what if someone clicks the search bar and doesn't know what it is? That icon will tell them."