The peculiarity of Berlin is that it has lower cost than many other German cities.
This is unusual, because typically you pay a premium for working in the capital of the nation. In Berlin, that is explained by the history (West Berlin was an enclave within DDR and the capital of BRD was in Bonn; Berlin was subsidized and had lots of relatively cheap housing which is not yet fully priced up 25 years after German unification).
If your customer is a German business, and you would like to visit them, it's very likely their headquarters are in West Germany, which is far away. So you don't have much country offices of multinational companies in Berlin. It's very different from other countries' biggest cities.
I guess it works better for pure internet companies that sell directly to consumers.
This is true, but I expect the situation will slowly change, and more business will concentrate in Berlin, because the federal administration is a big customer now that more of it is relocating in Berlin.
This is unusual, because typically you pay a premium for working in the capital of the nation. In Berlin, that is explained by the history (West Berlin was an enclave within DDR and the capital of BRD was in Bonn; Berlin was subsidized and had lots of relatively cheap housing which is not yet fully priced up 25 years after German unification).