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.
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 started to see another "offline sending support" every single day and can't understand the idea behind. If I'm doing something important, I would want to see that the form I filled is sent. I would never rely on an app saying "Oh yes, I saved this and will send it later for you". I am questioning this trend.