In addition to foreshadowing much of the plot yet to come in Romeo and Juliet, Friar Lawrence's first speech also connects many of the themes as well.
The beginning of Friar Lawrence's speech echoes the preceding balcony scene and its discussion of light and dark:
With references to "grey-eyed morn," "frowning night," "streaks of light," and "flecked darkness," we see this same theme echoed again and again throughout the play.
Likewise, we also see paradoxes and the use of opposites at work within the Friar's speech as well. For example, "The earth that's nature's mother is her tomb"
In addition to foreshadowing plot, Friar Lawrence's speech establishes his as a central character to the overall themes of the play as well.
In my opinion, Friar Lawrence is given such a long speech because Shakespeare is foreshadowing what is to come. You can see this in the content of the Friar's speech.
The Friar's speech meditates on herbs and plants and their dual nature. He points out how they can be used in good ways and bad ways.
This foreshadows what will happen with Juliet in particular. She will drink the potion that the Friar will make out of these plants.
I think the speech also metaphorically talks about how something good like love can turn into something bad (the tragedy that is going to happen).