Even disregarding roaming (e.g. travelling within the EU so roaming is free), having offline access makes it easy to plan what places to visit while flying to your destination.
It's also critical if you land late and are scrambling to your next destination and don't have service in this connecting country you are in and there is no wifi at all and your phone is on 10% and the ramshackle destination airport has exactly two plugs, one of them broken. I still print my itineraries and tickets and it's saved me plenty of times when my phone has failed me.
This. Offline support is a must. I got into the habit of screenshotting tickets because airline apps sometimes reload automatically, losing the ticket if you're offline.
