To what end? Surely he’s got enough good reviews by now to get real renters.
I don't have a second half-hovel (it's a duplex - look it up!) to rent out, so I'm no AirBnB host, but I'm sure it's to trick the algorithm. Last I checked he was Superhost, which has high reviews as prerequisite. And this pushes him to the top of the search results which definitely makes a difference in a big city where there are tons of AirBnBs in a small area. Plus I'm sure they have other algos at play where the most active and most reviewed hosts will be higher up in the results as well. Most people will search via map, where depending on the zoom factor only a couple of results will be shown.
And don't forget, Niki's "job" is literally SEO. From the other shit she has done (painfully obvious fake blog posts to generate clicks, the whole Torque Wheeler disaster), buying fake reviews for their AirBnB falls right in line.