What spam does
Spam is a common way for malware to spread and is used for Fraud. People waste tons of time sorting through there spam and it uses up bandwidth. Spam is often things messages you get trying to get you to give out personal information. In 2007 97% of all email was spam. Spam even appears on your phones. You can get text messages saying you one a competition and you end up getting SMS charges.
How to Avoid Spam!
Your fist defense is spam filters. After that you can do things like making sure not to reply to emails from people and companies you don't know. You can also create a separate email address for making accounts or even us a one time email.