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A page layout doesn’t have enough space in front of an input element to display a label, the function of the input is obscured, and a title alone isn’t visible enough.
Search and login forms are often subject to space constraints. There just isn’t enough visual space to display a label in front of the input field. Though without the label, the function of the input is obscured. A title attribute isn’t enough to fix the problem, because it’s rather hard to spot, requiring the user to mouse over the input and rest there.