It merges government and comparison data, realtor photos, and neighborhood and school data into programmatic pages. Hand-writing 500 million property pages at $1,000 each would be cost prohibitive, and technical SEO matters at its scale.
Zillow has huge reach among people browsing and starting a home search. It once launched a similar offering to Opendoor's, then became a partner, bringing the audience while Opendoor brings the transaction.
Real-time home values turned a dull listing site into a sticky product, giving buyers and sellers the same reason to return. Casey calls it a genuine innovation that others rarely pulled off.
Its market is mostly programmatic address pages, and SEO is worth about a billion dollars to it, so it should simply spend the money. At its scale SEO needs a large team.