I understood a little,but what is its purpose,especially in blockchain? I don't know a lot/anything about it.
Well actually there is nothing specific related to blockchian it came into discussion because of bitcoin's block hashing algorithm SHA-256, however conventional microprocessors used little-endian notation like Intel-836 and other followed. Big-endian is become most common in internet-protocols IPv4 and IPv6 that's why it's also called network byte order.