Your mastodon account is only good for loggin in the specific instance of mastodon where you made the account.
As for posting into kbin from mastodon, you can follow a magazine as if it was a user, for example i’m, @Mastur, whereas this magazine is @fediverse. That’s what i’ve understood so far, maybe if the federation was working properly i could test/try stuff further :(
They want to make sure you see ads so they can get revenue. A user that uses a website, even on mobile, could be using an adblocker. By forcing you to use the app they have full control of what ads you see and how often, thus incerasing the price at which they can rent that ad space.