At one point, Watson and Sir Henry have a trip on the moor Why might the author have written such a description?
I assume that you are talking about the episode in Chapter VI where Watson, Sir Henry and Mortimer are headed to Baskerville Hall for the first time. This chapter contains a description of their trip across the moor. I believe that the author wrote the description so that we would understand the tension that would face Sir Henry.
If you look at the description of the moor in Chapter VI, it is at once a beautiful place and one that is desolate and eerie. It's a mysterious place where you can be going along through fertile country and then suddenly come to wild places. The moor as a whole seems in some way forbidding.
By telling us this, the author shows why Sir Henry would want to live here. It is not only his ancestral land, but it is also beautiful and, in places, fertile. But the author is also trying to increase the tension in the story. He is making us feel that the moor is a dangerous place and that we can really have no idea as to what sort of bad or evil thing might happen next.
So, by describing the moor, the author gives us a better understanding of Sir Henry's motivations, but also creates a mysterious and foreboding mood in the story.