Generic functions in Julia can use types which have no reference to their definition using dependent compilation.

http://www.stochasticlifestyle.com/why-numba-and-cython-are-...

So you can work with types even when you've never seen their definition given how the compilation will occur with all of the pieces together instead of separately.

I usually profit from type definitions during debugging and memory layout work, so perhaps i don’t see this is a feature.

It's probably us that are using types in a weird way so it's my lack of explanation that's the issue. Types in Julia undergo multiple dispatch, so by passing a type into a generic function you can make the same generic algorithm run in different ways. So I use types to parallelize my code, calculate derivatives, propagate parameter uncertainties, and things like that. This talk from JuliaCon discusses a lot of the things for free that were developed by taking a type from one package and putting it into another:

https://www.youtube.com/watch?v=dmWQtI3DFFo