C++ - confused about the extern keyword

So I am trying to understand it and what it is used for, I
get the difference between a declared and defined
variable.

I'll try to explain what I am confused
at using the examples in the tutorial.

Example
1:

extern int var;
int
            main(void)
{
var = 10;
return
            0;
}

so here
I get that you cannot assign var to 10 because no memory was allocated to
it.

Example
2:

#include
            "somefile.h"
extern int var;
int
            main(void)
{
var = 10;
return
            0;
}

I'm
lost here. somefile.h would have something like int var = 0;to declare it correct? ut
then why would this program declare it again?