These novels are important as they can introduce new elements to the Star Wars universe that might later be incorporated into other media. They can also provide different perspectives on the events of 'Rogue One'. For instance, we can see how the ordinary people in the galaxy were affected by the events leading up to the stealing of the Death Star plans, which is something that the movie may not show as comprehensively.
They expand the lore. For example, the novels can show more about the relationships between different characters that the movie might not have time to fully explore. They also give a deeper look into the political and social aspects of the Star Wars galaxy during the time of 'Rogue One'.