Why does Sergeant Cuff intend to visit Mrs. Yolland again in The Moonstone?

Sergeant Richard Cuff visits Lucy Yolland to gather more information about her friendship with Rosanna Spearman, a maid who worked for Lady Julia Verinder. In particular, Sgt. Cuff wanted to know how and why Rosanna committed suicide. Lucy Yolland tells him that she was originally distrustful of people, but she warmed up to Rosanna and became good friends over the years.
Yolland also demonstrates her loyalty to Rosanna, even after her death, by stashing Rosanna's suicide letter for safekeeping and keeping it a secret. Initially, Sgt. Cuff finds Rosanna's death odd and wonders about the nature of it. However, after visiting Lucy Yolland, the truth is revealed about the suicide and other events that happened to Rosanna, as well as her feelings toward Blake. This allows Sgt. Cuff to get a clearer understanding of the larger picture.