How does the Duke's opening speech contribute to Twelfth Night?

The opening speech of Duke Orsino had contributed to Twelfth Night; it tells us the plot, setting and characters of Twelfth Night. "If music be the food of love, play on; Give me excess of it, that, surfeiting, The appetite may sticken, and so die." Duke Orsino described love just like a monster, which could destroy everything else in life. He was saying that love is high and everyone wants more of it, but it …

…shows his power. In additional, the background of the story should be with musicians playing musical instruments somewhere, in a big house maybe, as Orsino is a rich man. We are able to find out the plot, setting and characters of Twelfth Night from the opening speech of Duke Orsino. It gave us a brief idea on the story, at least we won't get lost. This is how the opening speech contributes to Twelfth Night.