Strange Behavior

As I mentioned before, I'm reading Ironhand's Daughter by the late David Gemmel. He's done something odd in the last quarter of the book and I'm waiting to see how it pans out. The heroine, her sidekick, and the reincarnated ghost of her father are making a side trip to recover some important Crown from an alien world through a magical gateway. So far this trip has taken up twenty or thirty pages and it's almost a completely separate storyline, with its own characters and situations. I can't see what the importance of doing this is. She needs the Crown in order to bring one of the highland factions into her army but that's not a good enough reason for a writer to send a story off in a totally different direction. When I get to the end I'll see how Gemmell ties it all together but for now it's just a little odd.