Developer portals play not only a technical role, they also play a somewhat obscure commercial role. We don't like to say that this is a commercial site, but actually it is also about promotion. You have to explain what you do – preferably not in a hard-core manipulative way. For that, you need more than reference docs. How to get started? What does that API do? A lot more than just the very technical aspects need to be explained.
A less well circulated purpose of developer portals is their being a trust signal. They engender trust by showing how much investment went into building that portal. They have to show that the API services documented on that portal will keep on existing and will get reliable support long term.