Service Workers give you complete control over network requests. You can return anything you want for the fetch event, it does not need to be the past or current contents of that particular file.
However, if the HTTP cache handles your needs, you are under no obligation to use Service Workers.
They are also used for things such as push notifications.
Documentation: https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API, https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API/Using_Service_Workers