Why do places want old film?

Perhaps you can answer me this.  In the back of some photography mags,
Columbus Photo Group or some such wants old film, any amount.  I have seen
others advertise for this too.  Can you tell me why anyone would want old
film, especially in this day of digital?  It can't be to test cameras for
repair as there are electric testers for everything, the little bit of
silver is not worth the hassle, so what else is there?  Got any ideas? 
Ric
in Wisconsin.