If you’ve done that well, they’re going to be asking themselves “How can I make this happen?”, which you answer in the last section of your letter by proposing an action they can take. “Call me at 鴔) 555 to arrange an interview” or “Let’s meet next Thursday to discuss this proposal” or “Order your copy today”. Don’t leave the action for your recipient to figure out. I think a lot of time we lack the confidence to ask for the action we want, and we give ourselves an “out” by leaving it unsaid, hoping they’ll figure it out themselves.
How do you know for sure whether or not to use a closing? One good test is to ask yourself whether this email is more akin to an instant message/text or a business letter. If you're giving your teammate a quick update on an ongoing project, a formal closing might not be necessary; if you're throwing your hat in the ring for a promotion, it definitely is required. When all else fails, and you're still not sure, err on the side of caution and include it. You'll never go wrong by being too polite and respectful.