Because it wraps an API of BVG, it includes all local traffic of Berlin & Brandenburg, as well as some long-distance trains running in the area. Essentially, it returns whatever data the BVG app shows, including realtime delays and disruptions.
Why use this API?
This API returns realtime data whenever its upstream, the API for BVG's mobile app, provides it.
No API Key
You can just use the API without authentication. There's a rate limit of 100 requests/minute (burst 200 requests/minute) set up.
This API has CORS enabled, so you can query it from any webpage.