Why are they restarting the key?

[4ET0pfjN]

Hi, I need to write a simple encryption function so I found this online to get familiarized w/ it:

void encrypt_data(FILE* input_file, FILE* output_file, char* key)
{
	int key_count = 0; //Used to restart key if strlen(key) < strlen(encrypt)
	int encrypt_byte;

	//Loop through each byte of file until EOF
	while( (encrypt_byte = fgetc(input_file)) != EOF) 
	{
		//XOR the data and write it to a file
		fputc(encrypt_byte ^ key[key_count], output_file);

		//Increment key_count and start over if necessary
		key_count++;
		if(key_count == strlen(key))
			key_count = 0;
	}
}

Can someone explain to me why we reset key_count to zero.

[kbw]

The key is a string. So it's using each character in the string in round-robin style to encrypt each character read from the file.