The messages they contain are usually untrue. A few of the sympathy messages do describe a real situation but that particular situation was resolved years ago. Hoax messages try to get you to pass them on to everyone you know. Most of the hoax messages play on your need to help other people. The biggest risk for hoax messages is their ability to multiply.
Most people send on the hoax messages to everyone in their address books.
Even if you only send to 10 people, after a week or two the hoax message could have grown to well over a million e-mails. This places strain on e-mail systems. Imagine if I send a single hoax e-mail to 10 others. The end result is a copy of my hoax message being sent to a huge number of people, which in turn costs organisations in the data traffic. This won't stop the 'spam' but does stop it from continually filling your e-mail inbox.