Diotima is not known to be a historical figure, and the way in which she is introduced suggests that she is almost certainly just a literary device. Plato's most familiar method of laying out philosophical ideas is through dialogue, and so he needs to find someone for Socrates to interact with. Agathon has just bowed out of the conversation, so Plato invents the character of Diotima in order to carry on the dialogue. Also, Plato wants to set Socrates up as the perfect exemplar of Love, one who is always searching for wisdom rather than someone who has wisdom. Socrates is famous for his remark in the Apology that he is only wiser than other men in that he knows that he knows nothing. Thus, Socrates himself cannot claim to know the truth about Love. Instead, Plato introduces Diotima as a prophetic figure who does know the truth about Love and who can lead Socrates toward it. Diotima is introduced to perpetuate the dialogue and to speak from a position of authority about Love.
We recall that Agathon's speech identified love in many way with himself. It was beautiful, wise, young, and the object of desire, according to his speech. Socrates reverses Agathon's claim, suggesting that Agathon, and the love he speaks of, is not Love itself, but is rather the object of Love's desire. As the passive partner in his relationship with Pausanias, Agathon is quite literally the "loved one."
We should note, then, that Socrates here sets himself up as the exemplar of Love. Love seeks wisdom, lives simply and is quite poor, he is tough and brave, according to Socrates' account, and all these qualities are also true of Socrates. Notably, Socrates calls Love a "lover of wisdom" which in Greek means quite literally a philosopher (philia = "love" and sophia = "wisdom"). Socrates is beginning a move that will be elaborated further by Diotima whereby the philosopher is the perfect exemplar of love, someone who is always seeking and never finding.
As such, this section further builds on the notion that Love is a relational property, one that holds between things rather than holding one thing in particular. Diotima builds a myth to rival that of Aristophanes, suggesting that Love is the go-between between mortals and gods, and that Love is the child of Resource and Poverty, always resourceful, but always in need. Thus, Diotima gives physical embodiment to a relational property by means of a myth. Considering a relation as a thing on its own, distinct from the objects it mediates between can be philosophically problematic, however. The idea of a relation as a messenger spirit makes for a compelling myth, and it does not particularly muddle the philosophy of the Symposium, but in more logically rigorous systems, it becomes difficult to account for relations as things in one's ontology.