1. The shortened address can be more easily shared with others. This is especially helpful for mobile text messaging.
2. Many allow you to track statistics on the number of visitors to that address.
A few examples of commonly used URL shorteners are tinyurl.com, bit.ly, Google's goo.gl, and Twitter's t.co.
Here's the big problem with using these in your emails:
Spammers use them too!
They are attractive to spammers because the address of the true destination page is masked by the shortener.
When you send an email, the receiving server scans the message to try to determine whether or not it is spam. One item that is commonly checked is any URLs within your message. Therefore, because spammers have damaged the reputation of these URL shortener domains, including them in your emails can hurt your deliverability.
The good news is that it's easy to avoid having to use URL shorteners in your emails. If the full URL is unsightly due to its length, send your emails in HTML format and hyperlink the full URL with some short descriptive text. If you require statistics on visitors to your links, JangoSMTP includes a click tracking feature that can be branded with your own domain name.