If your intent is to persuade the recipient in some way, whether it's to invest money, give you a reference, hire you, partner with you, or fix an issue, create a compelling case for your cause. If, for example, you want the reader to sponsor a charity event, identify any overlap with their company’s philanthropic goals. Convince the reader that helping you would be mutually beneficial, and you will increase your chances of winning their support. Keep your closing paragraph to two sentences. Simply reiterate your reason for writing and thank the reader for considering your request.
Consider the daily life of whomever you’re sending a letter to _how many other things might they have on their mind when they read your letter. Maybe they have a meeting in five minutes. Maybe yours is one of 200 letters they have to wade through. Maybe it’s the end of the day and they’re worn out. Giving them a concrete action to take makes it all the more likely that it will “stick” _ they’ll add it to their todo list or their calendar, they’ll pick up the phone, they’ll check out your website, or they’ll respond.