It is both! While it does add two new routes, it primarily gives clues to figuring out the overall mystery of what is going on and answers a lot of questions asked in Book One, including a lot about Inigo's identity. If you are mostly interested in male ROs, because Lana's route is platonic only, you can romance anyone whose romantic routes you have previously completed, including Kay and Clive. :)
Can you please clarify what you mean by this? I’ve been trying to romance Shannon but I keep triggering Lana’s route instead on accident and I don’t know what it means, especially now with seeing what you’ve said about “previously”. Am I unable to pursue Shannon romantically while on Lana’s route until I’ve successfully triggered her route and did it?
Yes, to romance Shannon in Lana's route you'll need to have completed at least one of Shannon's romantic endings in her own route first. If you're having trouble getting onto Shannon's route, there's a walkthrough available here!