Why does/did C allow implicit function and typeless variable declarations?

Why is it sensible for a language to allow implicit declarations of functions and typeless variables? I get that C is old, but allowing to omit declarations and default to int() (or int in case of variables) doesn't seem so sane to me, even back then.

So, why was it originally introduced? Was it ever really useful? Is it actually (still) used?

Note: I realise that modern compilers give you warnings (depending on which flags you pass them), and you can suppress this feature. That's not the question!

Example:

int main() {
  static bar = 7; // defaults to "int bar"
  return foo(bar); // defaults to a "int foo()"
}

int foo(int i) {
  return i;
}