c++ - In C, importance and usage of extern storage class specifier

Actually I want to know the importance of extern.

First I wrote some code :

file1.h

extern int i;

file2.c

#include
#include "file1.h"
 int i=20;
int main()
{
   printf("%d",i);
   return 0;
}

Now my question is that: what is the use of extern variable when I have to define the i in file2.c, declaring in file1.h is in which way useful to me .

sudhanshu