They use it to make their web sites more interesting, more attractive, funnier, to make a point, or just because pictures break up the text. Teachers put clip art in class letters home.
Principals pop it onto the monthly newsletters to parents.
We all toss a clip art piece onto a memo to catch the reader's attention. Students use it to inspire writing projects or as decoration. . Usage of clip art is widespread. Everyone uses clip art but most people complain about it. And of course, there is the problem with copyright.