Because it is helpful. If you have a few drafts going on, or are busy working while playing some games, it's easy to lose track. Since it takes less than 30 seconds to send a sitemail, it's just a courtesy to one's fellow owners and can prevent someone from missing out on a player he might have wanted.
EDIT: I just timed it for the heck of it. Took me 21 seconds to go from this page to the first, copy the three picks after my next one, go into sitemail and send a note.