I think your question has a presupposition that is already wrong:
It is not the language, and even less a language's grammar that makes orthography rules.
Orthography is a set of prescriptive, more or less static and sometimes seemingly arbibitrary rules that are about how to transform language into graphemes, but not an inherent part of a language itself.
This is why it doesn't even really make much sense to ask a linguist that question, because linguistics doesn't care about commas.
To still make an attempt to answer your question:
Orthography rules aren't totally useless - how would you use a dictionary or make a Google search if there was absolutely no consensus how to write a word? - and when it comes to arbitrariness and unpredictability, English can be claimed to be significantly worse than German!
The main point is probably that English word order is much more restricted, while German allows for more variance in word order especially between main clauses and subordinate clauses. In speech this can be disambiguated by intonation, while this is obivously not available in written langauge, which is why you make use of punctuation instead.
The rule is that in general, German subordinate clauses need to be separated by a comma, while this doesn't hold true for English. In German it is often more complicated to determine the word order and relationships especially in main clauses because you don't always strictly have SVO (or SOV, which would be the default word order in subordinate clauses) there, and that's one of the reasons German orthography makes more use of commas.
And if the general rule is that every subordinate clause needs to be separated by a comma, then this is in fact easier than making a large list of exceptions for those cases where it seems a little bit more "useless". Everything which is a subordinate clause is put a comma in front of, and this is why this rule totally regularly applies in your example.
In that way, the rule is not as nonsensual.
But again, orthography is not really a concern for linguistics. See this question for some more comments as to why orthography is not an inherent part of natural language itself and therefore not what linguistics traditionally deals with.