I respectfully agree with the thought that you can always use browser to access the forum. But if this is the only point then I think there was no need of having Quora application because we can also read quora on browser.
Quora and other apps like Facebook, have their own app because they are very much advanced in web structure, so making their own specialized app will make the user experience much better.
But this forum is just a simple forum board. There is no need for an app. Rather, there is need for better mobile experience, because the piece of forum script they use is more than 12 years old.