A missing feature pops up every now and again in Gmail. We can't delay emails. There are certainly times when it would be useful to send a finished email not just yet. I may send emails when it is not the best time for the recipient to read or respond. And when they return to their email later, mine will be one of many and not receive all that careful consideration. I think this can be a useful feature.
Boomerang1 adds delayed emails and much more. For example, it can return conversations to the inbox if no one has responded in some time.
1This is the link for the Google Chrome extension.