Possible use cases

Website optimization If you're running a (large) website, jobs that do heavy lifting should be taken away from the user interface and scheduled to run on the machine separately.

Log parser Continually scan logfiles and import critical messages in your database.

SMS daemon Read a database queue, and let your little daemon interface with your SMS provider. If it fails, it can easily try again later!

Video converter (think Youtube) Scan a queue/directory for incoming video uploads. Make some system calls to ffmpeg to finally store them as Flash video files. Surely you don't want to convert video files right after the upload, blocking the user interface that long? No: the daemon will send the uploader a mail when the conversion is done, and proceed with the next scheduled upload