Diller is an aggregator of personal data. He buys companies that add to that and that are easily monetized -
travel, real estate, finance, dating, etc.
I suspect that buying a search engine is more about the data that it yields than the ability to give his sites a boost. AJs market share isnt enough to give IAC that much of a boost if they did massage the serps a little, but the data on its users is probably a large enough sampling to extrapolate web wide. Knowledge (information) is power, right?