Speed is pretty important.

Compiler telling you about a bug because you have a typo in your code (as opposed to blowing up at runtime, sometimes in a way that you won't notice) is also important.

Struct definition is also a documentation of your data. Another developer (or you a month from now) doesn't have to go back and re-read, say, Redit API docs. He (or you) can just re-read definition of the struct and know all the data that a given JSON request provides. That's nice.

An editor can parse the struct definition and give you auto-complete, which is also nice.