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.