Most emails contain one or more calls-to-action. You want people to do something once they receive an email. This is why you may want to add links to your mail, because it makes it easier for your recipient to visit your website, to order a product, or to download a whitepaper.
Links in your email also enable you to measure the conversion of your email. This allows you to analyse how many times a link was clicked. You can use this information to make your next email even better. If you give every link a unique title, you can measure every link separately.