Almost all the mail systems can sort in either direction (first received or last) and indeed by other criteria. However many people seem to get caught in the trap of a big mailbox, and want the new messages at the top, because if you respond to new messages first, you seem very responsive to them, and if you delay 10 day old messages another day, it's no big increase.

This is a bad habit of course, but many people have it, and so that's why we want our software to help with it.