The description of Maro, son of Euanthes, occurs in Book 9 of Homer's Odyssey, a book mainly devoted to Odysseus' encounter with the Cyclops Polyphemus. As mentioned above, the notion of "xenia" or hospitality to strangers and reciprocal gift exchange was a key element in Greek ethics. Zeus is sometimes referred to as the "god of strangers" and responsible for rewarding those who follow the dictates of xenia and punishing those who do not.
In contrast to Maro, whose gifts form the basis of a civilized interaction between host and stranger, Polyphemus violates the dictates of hospitality by killing and eating his guests. The detail that it is Maro's wine Odysseus uses to get Polyphemus drunk links the example of the positive hospitality of Maro to the negative one of Polyphemus.
In blinding Polyphemus, however, Odysseus himself violates the rules of hospitality, thus provoking the revenge of Poseidon.
The encounter with Maron shows just how important hospitality (also called xenia, meaning roughly the duty of a host to a guest) is to the Greeks. They spare his life when they are destroying the entire rest of the city; he gives them wine in return. An exchange of gifts or services binds the two parties, and will bind their offspring as well. They are strangers no longer.
Greek hospitality also shows a certain interest in merit-making. They were inclined to even receive guests who were beggars and would not have anything to offer in return.