The most frequent reason for offline mode I heard is "using app while being off grid" which is quite ironic to me since it's when people suppose to do something else. This excludes offline-only apps like calculator.
You might be at a remote site, or on a plane with no Wi-Fi.
But, yeah, if those are common use cases for your app, you should absolutely be designing for offline.
Your job may literally involve entering data at a remote site with sketchy-to-no internet (mines, farms, geological surveys, etc).