They expand the lore. For example, they can give backstories to characters we see in the movies. 'Darth Maul: Son of Dathomir' novel gives more details about Maul's life and his connection to Dathomir.
Star Wars lore novels contribute a great deal. They fill in the gaps between the movies. The novels can explore different planets and cultures in more depth. For instance, the 'X - Wing' series novels explore the lives of the pilots and the workings of the Rebel Alliance. They also allow for different interpretations and perspectives of events, which enriches the overall understanding of the Star Wars universe. Many novels introduce new characters that later might be incorporated into other Star Wars media, like the Clone Wars series.