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> They help define and navigate the representation of composite things as opposed to just having dynamic nested maps of arbitrary strings.

What would you say to someone who thinks that nested maps of arbitrary strings have maximum compatibility, and using types forces others to make cumbersome type conversions?

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If the fields of a structure or the string keys of an untyped map don't match then you don't have compatibility either way. The same is not true for restricting the set of valid values.

edit: To put it differently: To possibly be compatible with the nested "Circle" map, you need to know it is supposed to have a "Radius" key that is supposed to be a float. Type definitions just make this explicit. But just because your "Radius" can't be 0, you shouldn't make it incompatible with everything else operating on floats in general.

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