What is the purpose of placing the most significant byte first? When the SHA algorithm states "A block of 128 bits is appended to the message. This block is treated as an unsigned 128-bit integer (most significant byte first) [emphasis added] and contains the length of the original message (before the padding)[,]" what is the purpose of placing the most significant byte first? 

For example, if we have an integer of 24, which would be 00011000, and we therefore need 15 bytes of 0's to complete our 128-bit block, does this mean that 00011000 comes first followed by 15 bytes of 0's? It seems like this must be what they're saying. 

I'm just confused why this byte needs to be first...

Source: Network Security Essentials (6th ed), William Stallings, p. 72
$24$ 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.