I recently purchased a date stamper for my kid's art work and school work. I saw that Michelle at Delightful Learning used one on her kid's school work and it seemed like a good idea.
Not only is it easy to use and a good way to keep track of when they make things, it also gives me that old-school-library-lady-stamping-a-due-date-card kick.
Which is probably the real reason I do it. :)