Some folks I know found that the best time for an email to reach someone's inbox to give it the maximum chance of being read was 8am Tuesday. Then it'd be at the right place in their inbox to get read! Wanting to catch execs for a sales pitch in the morning when they are fresh, rather than at 5:30pm when they're likely to just bin your mail.

Second, perhaps you don't want them to see it now, or reply to it now, but after some other event.