Spot the Signs. According to Alison Green, who writes the blog Ask a Manager, this is common practice for some industries, especially if you're leaving to work for a direct competitor. However, in those situations, you'll likely know it's coming and can prepare accordingly.
A resignation letter can also help your boss and your company start the transition process, get a replacement for you approved, and start planning internally, etc. The paper trail is beneficial all around. So what should go into your letter? Let's look at some examples of what to do (and perhaps more importantly, what not to do).