Both this answer and this article suggest creating a separate import called a "resolver" that imports the service that retrieves my data. I feel like it would make more sense and be less code to just modify my services to implement Resolve.
Angular's docs even go so far as to name their resolvers services, eg. crisis-detail-resolver.service.
Why do resolvers need to be separate from the services they use? Or is it OK in some instances to make your services implement resolve?