Tum sē M. Valerius tribūnus mīlitum obtulit, et cum prōcessisset armātus, corvus eī suprā dextrum bracchium sēdit.
Then Marcus Valerius, a tribune of the soldiers, presented himself, and when he had gone forward armed, a crow sat above his right arm.
Why is eī in the dative? Also, why is it in this sentence at all? It seems superfluous.
It is superfluous in the sense that it’s not absolutely necessary to the sense. But this use of the dative is quite regular. Lit. “sat for him above the right arm”: the dative gets “him” into the clause, and much more idiomatically and unobtrusively (slipped in as second word) than bracchium eius would. If you want to pin a label on it you can call it a “dative of advantage or disadvantage” or a “dative of interest” or an “ethical dative” or a number of other things; the classification is rather arbitrary.
I was listening to Schubert, and in Winterreise’s finale, Der Leiermann, we have
I took, hopefully correctly, ihm here as datiuus ethicus. I hadn’t realised that German, too, has ethical dative.
First you’d have to establish a taxonomy for datives in German. Can you simply take over grammatical terminology that was invented for Greek and Latin and is only the product of a largely arbitrary and unstable classification system made up by old-fashioned grammarians anyway? I wouldn’t like to give up “dative,” since the case system is still built into the German language itself, but anything else—particularly function—is up for grabs.
And as for “datiuus ethicus,” is it anything more than what latinizing grammarians call a dative when they have nothing else to call it?