For an average joe forum admin, it's not going to be anything special, since the majority of people will never use it.
For people that create modifications or have applications they'd like to interface with UBB then it will be much easier. For example, a Joomla bridge has been asked for quite a bit. It's pretty tough right now since everything would need to be coded from scratch and if things changed on the database side, the bridge might no longer work.
With an API, it will be much easier since the majority of things will just require a simple function call to the API. If the database is changed or modified in an update, the API would be updated as well, so the bridge would continue to work.
