Sometimes as a module developer you need to have some code execute periodically, like maybe every day or even once a week. This might be to optimise an external system, or pull in some external data or to compute some statistics every day etc. Drupal provides a very simple way of doing this: hook_cron but if you want your code to only execute say once a day or after 6pm only, then you have to add extra logic to your implementation. However, the Job Scheduler module provides a nice framework for setting up and running these periodic functions.