Why do I need to flush my I/O stream to get the correct result?

Why the code below does not work? I mean, it shows all kinds of weird characters on console output.

#include <stdio.h>
char mybuffer[80];
int main()
{
    FILE * pFile;
    pFile = fopen ("example.txt","r+");
    if (pFile == NULL) perror ("Error opening file");

    else {
        fputs ("test",pFile);

        fgets (mybuffer,80,pFile);
        puts (mybuffer);
        fclose (pFile);
        return 0;
    }
}

However, the code below works well.

#include <stdio.h>
char mybuffer[80];
int main()
{
    FILE * pFile;
    pFile = fopen ("example.txt","r+");
    if (pFile == NULL) perror ("Error opening file");

    else {
        fputs ("test",pFile);
        fflush (pFile);    
        fgets (mybuffer,80,pFile);
        puts (mybuffer);
        fclose (pFile);
        return 0;
    }
}

Why I need to flush the stream in order to get the correct result?