Are the non-human but almost human races really necessary? Well it depends on what you mean - would you want to get rid of them completely, or are they not bizarre enough to serve a purpose?
After all, almost no fantasy or sci-fi book is about alien races, not really. They are about examining human traits, human conditions, human potentials, and human stories, because those are the stories people connect with, and, well... write what you know is generally given advice for a reason. At the same time, they exist in the realm of the hypothetical - they often want to tweak human culture, human history, even human physiology, and play a game of "what if". What would humans be under these conditions? Sure, its impossible. Heck, we probably wouldn't even call them human anymore... and then boom, a race is born.
These races are also useful in making commentary on modern day real-life race relations, they often serve as important plot points (shared ancestry is popular), they give a world a feeling that there is more than just people. I mean, as much of a stretch as humanoid aliens might be, I can imagine it would be a lot more so if they were ACTUALLY human, don't you think? Or we could have no aliens at all, but that certainly changes things! So there's plenty of reasons to want things that are alien, to explore concepts humans don't really let you explore, but you want them to be familiar enough, comfortable enough, that you don't need to interrupt the concept to explore what the alien actually IS.
These regular human-variant races aren't always true, though - look at the alien's in the Blue Adept series (Piers Anthony's better series) for example. Enders Game. Starship Troopers, Ur-Quan Masters. Look at many of the threads about this very topic that happen in Creative. Heck, there are even some popular player races in my own fantasy/sci-fi campaigns world that are very un-human - fridge-sized telekinetic beetles, six-armed dogs (that's in addition to the four legs), giant cybernetically enhanced scorpions, floating land jellyfish-squid. And I like their presence in all those works, because they really help explore concepts and ideas you might never see at all if you stuck to normal humans!
From good authors, you'll get weirder aliens when the serve the purpose of the story, plain humans when aliens won't help, and humanoid aliens when you need to explore alien concepts within a familiar framework.