Have you ever tried to send someone a link to an item from Amazon.com?
Sometimes links can be so long that they break apart in email programs and no longer work. Why do they break? Because a URL cannot have any spaces in it. When a URL spans across multiple lines in an email message, some email programs will add a hard return that breaks the link in two.
Best of all, the new link never expires. TinyURL has a huge database where they store the old and new URLs together so that if you click on the TinyURL link in 2 years, the link will still work.