That is a very good question. I think he did because it was a way to make the book seem more like reality. The book is in the future, and many things have changed. There are televisions where you can live in, fireman who start fires instead of put them out, a mechanical hound which fights crimes, and then the people; all the live for is to be happy. It's like the nature is one of the best and true things still left, and its Montags 1st experience. The whole end of the book is mostly imagery of the river and the land around it. It takes the book and goes from the future where nothing is the same, to now with rivers, meadows and the sky. In the end, Montag wants to go back and rebuild the city and make things go back to how they were.
I think he brings in nature to bring some light into the novel because everything else just seems so dark like Montag's house and his bedroom. When Clarisse talks about nature though, it gives a tone of happiness. I also think that the reason he has Clarisse talk about nature is to show that not everyone was mesmerized by their parlor, and that there were still people who took time to smell the flowers and take walks down the street rather than drive everywhere in such a rush. The thougt of nature just brightens the novel.