You can book some trips that go through Germany but don't actually start or end there (e.g., Amsterdam-Göteborg).
Conversely, there are some trips that are best booked online on the web site of the national railway of a third country. For example, the site of the NMBS (Belgian Railways)[0] knows about some Dutch fare reduction cards that the DB (Germany) doesn't know about, while it also knows about more German stations than the NS (Netherlands) does, so some (admittedly fairly obscure) tickets from the Netherlands to Germany can only be bought online on their site.
