- Power - you can do arbitrary checks on the data, static type systems are quite weak in what kind of properties they can verify (far from turing complete)
- Flexibility to do checking at where you want at runtime (eg check data at API boundaries)
- Ability to treat schemas as data, generate them, output them as data and share between systems (for example in databases), throug conversions possible to interop with other platforms (eg json schema)
- loose coupling to your programming language, are just libraries