Not everybody lives in large urban areas where Internet is an almost guarantee. I myself live in a remote area of Ontario where coverage can be spotty at times. It's nice to have a fallback to offline storage when I don't have access. Not everybody is willing to pay for a data plan as well.
Yes, I have experienced bad internet connection before too. If I'm filling a form, when I click "Save" button I would expect it to save, if my connection is bad, it would either say "failed" or the loading wheel of death would go forever which would make me move my computer or phone to somewhere else to get better reception.
Saving to offline storage and sending via service worker when the devices comes back online sounds horrible because there is no way to explain a user that the form they sent will be processed automagically when they are back online.
For sure it's painful. I experienced that with a project we have here. It's a submission portal for students at a remote area college. Students there typically don't have reliable access to the Internet. When they go to submit their answers to their assignment, if they do not have a connection then we have to save the information locally and submit when they come online.
It's also nice because now the students can view their list of courses and assignments without having Internet. I wouldn't say there is a use case for every app that markets it but there definitely is a market for offline availability in some cases.
I totally understand and support offline reading which makes a lot of sense since you want to access something you had already downloaded. But I believe sending something later and relying on an app for an important form wouldn't make anyone comfortable.
