Too many companies view app development as a one off activity that can be outsourced (external or internal within the company) and then, once complete, not require the developer any more. However, most non trivial apps need ongoing support of some kind.
Things break. New phones are released and new versions of operating systems cause things to break or misbehave. People, perhaps working on things that the app connects with, change things that cause the app to need to be updated.
Things need investigating. Often there’s a need for 2nd line support to work out what’s happened – even if it’s ‘user error’ and not a fault of the app.
Things need improving. Almost every non trivial app I have worked on has undergone small changes after users have fed back how it can be improved.
Review criteria evolves. What passed Apple’s tight scrutiny a year ago might not pass now. Changing your app in any way might cause it to have to be further updated to meet tighter new criteria.
Factor in time and cost to update your app after it has been initially released. Talk to your app developer before this happens so you have some arrangements in place.