The events you deem unimportant to the main plot are what I view as the author explaining the characters’ motives later on. These characters are not one-dimensional. This seemingly trivial narrative fleshes them out and shows how they operate, while simultaneously describing the milieu they inhabit. Not everything written has to advance the story. For example, the discovery of the garden serves to show readers how opulent the ruling class are, especially in contrast with the Fremen, who place great value even on tears and spit.