I never understood this, how is it possible that a developer calling a function doesn't test if they are calling it right. Once they test it, how does the additional compiler test matter?

Unless of course your calling function is mutating the value of a variable across so many types that it's impossible to realistically test it. It would then make sense to use a type to prevent the variable from being something it shouldn't.

But most people, don't have that situation most of the time. Even in a big application.

May be that explains why something like Python has such wide adoption.