As their names imply, @thehumanbot and @humanbot are humans who upvote posts that have been recommended to them once they check them for quality. They're actual curators who are motivated to provide the kind of service that you've written about - providing good upvotes for good posts.
@thesteemengine encourages engagement and gives one upvote per day to members who upvote and make thoughtful comments on the posts of two other members. Junk posts aren't tolerated, so the engagement has to be genuine.
That's good then. I'll keep on supporting my set of delegatees for now