Purpose: This is needed to support in app prompts to upgrade from a basic to a premium plan, a fairly mainline pricing pattern these days. Since the subscription can only be upgraded in the app store, there needs to be a way to send the user our of the flutter app to the app store, just like with the initial purchase.
I don't believe showManageSubscriptions or the Android equivilant exists in the chargebee library.
I'm listing as a feature request, but it really is an issue, as offering the users the ability to manage their subscriptions or upgrade is an essential part of managing mobile subscriptions.