It's August. I don't want to travel to Berlin in December, I want to travel today or tomorrow.
However if you do want to book stuff months out, advanced purchase tickets are a staple of the airline industry. From Berlin you can get to a host of locations for about €20 each way, including Venice, Milan, Budapest, Madrid etc - far further than a domestic train.
Yeah I know there are some methods, but in my case it's in September, 3 weeks out and for reasons out of my control I could not book in May already. I'm not even taking "spontaneously tomorrow", and last I checked even 1-2 months in advance can still be more like 60 rather than 20 :(
Of course, the reliability of Deutsche Bahn is another question.