Standard "good, better, best" pricing psychology. The high priced model make people who can afford the lower priced models feel like they are getting a better deal. Thus more sales. The low priced models make the people who can afford the top product to feel like they are getting something more special and differentiated. Thus more sales.
Same with grocery store products. Take 3 nearly identical products on the shelves and price them low, medium and high, instead of the same, and total revenue goes up. People buy more just because someone played with the numbers and used the research results.