Exactly, the company needs to know whether to tell the other candidates in queue to go away, or pause for a while. They can't wait indefinitly so an expiration

Exactly, the company needs to know whether to tell the other candidates in queue to go away, or pause for a while. They can't wait indefinitly so an expiration on the offer is reasonable. That said, don't take the expiration date too seriously, it's not a hard deadline. If you want a bit more time just say so.