Those popup balloons that give a dictionary explanation for every single word on a webpage. What kind of 'usability', or feature do the webmasters of these sites think they are providing?

On some sites, at least, the words are highlighted with double underlines or similar - which is annoying enough when merely hovering over the word whist reading the article invokes a popup balloon.

But on some sites accidentally clicking the mouse over any single word on the page (even words like 'the') will invoke a popup dictionary of some sort.

Is there a more annoying website feature?