Howl's Moving Castle raises a lot of questions: Who or what is Howl? Why does his castle move? Is the castle going to be the general setting of the book, or is it going to be the goal of a long journey? The title immediately gets our interest because it unites two words—moving and castle—that seem like they should never go together.
But beyond the fact that this title is exciting stuff, the name of the book also markets it as a fantasy novel. After all, who is named Howl outside of a fantasy novel? And these days, it is pretty rare to find people living in castles, moving or otherwise. If the novel is about a castle, then it's probably either historical fiction or fantasy. And once you throw in the weird adjective moving, then it seems much more likely that this novel is going to involve magic, as opposed to realistic portrayals of life in medieval times (where the castles pretty much never move).
The title of Howl's Moving Castle makes us curious about the plot and setting of the book, but it also strongly hints at the kind of book this is going to be. All that, and it's catchy too.