The story in the anime is linear, obviously. The difference here is that since you'll be pursuing routes, you'll see different developments and endings, for example, Iwasawa is not a love interest in the anime, but here she'll be one. For people who haven't watched, it's probably going to be another VN, but for those who watched, it'll serve to tell us more about the characters, since 13 episodes was not enough for that. I don't think you HAVE to watch the anime, but if you do so, the novel will surely be way more enjoyable.