The symbolic importance of Alcibiades as Dionysus works on a number of levels. Most importantly, he appears at the end of the drama to celebrate Socrates. The dramatic festivals were given in honor of Dionysus, and Agathon's victory would thus have been an homage to him. It is significant, then, that Alcibiades enters in order to wreath Agathon with garlands as the victor at the festival. But when he sees Socrates, he insists that he take some of Agathon's ribbons and place them on Socrates, since Socrates has a way with words that outmatches anyone's. In this sense, we could see Socrates as stealing Agathon's victory from him by besting him in the series of speeches. At 175e, Agathon suggests that Dionysus may judge who is the wiser between them, and, in the form of Alcibiades, Dionysus does just that at the end of the dialogue.
The dialogue itself, of course, is also a drama, and Alcibiades' appearance at the end could be read as the appearance of the god to celebrate the drama that has just taken place. By having Socrates best Agathon, Plato suggests that philosophy is greater than tragedy, and Socrates' celebration by this symbolic Dionysus suggests that the philosophical drama we have just read is greater than any tragic drama. Also significant is the fact that Alcibiades/Dionysus enters after the speech of Socrates/Diotima, missing the final mysteries which Diotima reveals. Perhaps these gifts are reserved only for philosophy, and tragic drama, no matter how good, is bound to miss them.
Finally, we might also note that in the speech of Diotima, Socrates suggests that the gods most love one who knows true Beauty. Here, the god Dionysus seems to be valuing Socrates' wisdom above that of all the others.
There is also a great deal of importance in Plato's choice of Alcibiades the man, outside his symbolic role as Dionysus. Alcibiades was a great politician who was involved in a number of scandals that led to Athens' loss to Sparta in the Peloponnesian War. Socrates was a known associate and friend of Alcibiades and a few other Athenians who had fallen out of favor, and it is suspected that these associations were the real cause of his trial and execution. In order to redeem Socrates' name, Plato carefully points out that Socrates was not responsible for Alcibiades' political misdeeds. Alcibiades notes here that Socrates was always persuading him to abandon politics and to concentrate on his ethical development, but that Alcibiades refused to listen. Thus, Plato renders Socrates blameless of anything Alcibiades may have done in the political arena.