I have a theory about the exact purpose of the Winter Knight and why Mab wanted Harry for the job.

We know the winter court exists to largely fight off the outsiders and defend the universe. Mab is the de facto ruler of that court as its winter queen, one of the winter lady jobs as we now know is tasked with gathering soldiers for the war, the role of mother winter is unexplained but presumed to be powerful. The role of the winter knight as largely been shown to be Mabs hitman and soldier.

But I believe theres more, so much more. Power has limits and Mab is one of the most powerful beings Harry knows. I believe Mabs limit is that she cannot use her magic to outright kill a mortal not owing their life to her. This presents a problem in the form of the Nfected, mortals corrupted by the outsiders working to let them in.

The solution: Acquire a mortal with power of their own, give them a portion of your power to boost them and send them to assassinate and stop the Nfected and the plans they make.

Why Harry? As we see by LLoyd Slate and the other winter knights they are "the worst of humanity". Power corrupts and so because the winter knight mantle amps up your aggression and baser instincts to make you more focused on survival and the fight, it corrupts weaker men. But Harry has spent many years fighting of the influence of dark magic, resisting the lure of a fallen angel in his head and has turned power away despite its benefits to him for fear of its corruption. His whole life outside of magic has proven him to be strong of will as well. Harry is one of the few mortals able to resist the mantles power enough to be able to use it but keep his mind intact making him more than "Mabs cruise missile". He is also an accomplished wizard and a starborn to boot. He is the perfect Winter Knight for Mab, hence why she laboured for so long to keep him alive after he tried to cheat her, its also the unspoken other reason why she sent Harry to assist Nicodemus. She wanted Harry to acquire the holy relics to add to his arsenal for the battle to come