My company keeps a req open on the off chance somebody in the domain (CAD data translation) will see it and apply. It's a small enough niche that if somebody with the right skillset comes along we'd like a chance to hire them, even if we're not in dire need of people at that particular moment.
In other shops it is sometimes used as a safety if someone spontaneously decides to go.. one local place puts out lots of feeler reqs around bonus time (as that is when most jump)