A wide variety of sources queryable through standard SQL, including some not found in alternatives. The competition for querying the common data sources is, as you note, stiff! We want to do those well, but also chase the long tail of more obscure systems, interfaces and formats that are out there and still contain stuff people are interested in. Even people who'd like to run query or two before they're required to declare a schema, though admittedly this latter trick is helpful for exploring but no magic wand once you need start doing computation (and data types really do become indispensable).