Unclear effect of `add_function` type parameter

[kapcod]

In all the add_function examples you pass :numeric type, but there's no clear explanation why this type needed and how it affects the functionality. I experimented with this and couldn't see any effect it does. There's no type casting in case I wrote :numeric but function receives or returns string. I could even pass some garbage as type and it still worked exactly the same.
If this type is not needed, maybe you can drop it (or make optional for back-compatibility)?
If it does anything, maybe add a few lines of documentation?

Thanks