Why should i put SEEK_SET twice

I want to modify some vowels of a file by "5". The following code works. However i do not understand why should I put fseek twice.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

void print_file_contents(const char *filename) 
{
    FILE *fp;
    char letter;

    if((fp=fopen(filename,"r+"))==NULL)
    {
        printf("error\n");
        exit(1);
    }

    fseek(fp,0,SEEK_END);
    int size=ftell(fp);
    rewind(fp);

    for(int i=0;i<size;i++)
    {
        fseek(fp,i,SEEK_SET);
        letter=fgetc(fp);
    
        if((letter=='a') || (letter=='e') || (letter=='i'))
        {
            fseek(fp,i,SEEK_SET); // WHY THIS FSEEK ?
            fwrite("5",1,sizeof(char),fp);
        }

    }

    fclose(fp);
}

int main(int argc, char *argv[])
{
    print_file_contents("myfile");
    return 0;
}

In my opinion, the first fseek(fp, i, SEEK_SET) is used to set the file position indicator to the current character being processed, so that the character can be read using fgetc. Hence, the cursor is updated everytime so there is no need to add an other fseek(fp,i,SEEK_SET);

>Solution :

The fgetc advanced the file position; if you want to replace the character you just read, you need to rewind back to the same position you were in when you read the character to replace.