If you want to read (say) 50 bytes from a 100 byte array starting at byte 75 your code will need to know not to try to read array[100] (which doesn't exist) and use array[0] etc instead.
If you use modulo it will return (say) 98, 99 while the 75-125 count is under 100 and will automatically give 0, 1 2 when the count exceeds 100.
...R