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).