Well, the first line often acts as a hook. In the case of Dune, it could be drawing readers into the complex and unique world of Arrakis. It might start to introduce the harsh environment or the political intrigue that is central to the story. By starting with a strong first line, it grabs the reader's attention and makes them want to know more about this strange and fascinating world. It could also introduce a character in an oblique way, perhaps through their perspective on the world that is being described in that first line.
The significance of the first line can set the tone for the entire story. It might introduce a key concept, a sense of place, or a mood that pervades the Dune universe.