What is the purpose of placing the most significant byte first?
$24$ expressed as a 16-byte hexadecimal number is
$$00\;00\;00\;00\;00\;00\;00\;00\;00\;00\;00\;00\;00\;00\;00\;18$$
The most significant byte is the leftmost. $00011000_2$ comes last, not first.  

It makes no difference to the security of the algorithm whether the most significant byte comes first or last. What is important is that everybody agrees on this point.