Zillow wins by buying an asset it has more information on the true market value of, then providing an easier, economies of scale on the associated services (inspection, legal, repair, etc), then being able to offer a standardized product to home buyers.
Buyers win from taking a lot of the uncertainty out of a transaction. And from possibly lower fees if Zillow decides to rebate a portion.
Let's not forget there's a standard 6% commission in US real estate transactions... most retail would kill to start with a 3% margin.
If Zillow has high confidence a home is underpriced, it makes sense to time arb that into profit when it can match with a buyer.