Do you also have an android app? What I'm asking I guess is not why do you need the AppStore when you are on iOS. But more why do you have to be on iOS in the first place?
I guess I was a bit misleading in my original comment. 98% of my users come from iOS and Android organic search results. I haven't been able to find any profitable non-store marketing channels.
It's not Apple or Google. It's users who are paying.
If they want access to the market then it's a "Want to be" not a "have to be" because of the opportunities apparent.