Lots of spammers are trolling lots of web sites using spider engines similar to what search engines like Google use, to find every email address they can. These spammers then trade their gatherings amongst themselves. If you have a million fresh emails, you can trade them off to get another million in exchange, often from many other spammers, and end up with tens of millions as a result.
Then those who actually send the spam try to come up with any and all clever subject or message to get you to act. They might try to get you to open or run an attachment, or access a web site. Sometimes it is as benign as knowing your address works and they can sell it for more money. Or it may be to infect your computer with spyware to pop up ads on you all the time, or turn your computer into a spamming zombie, or collect your private info like credit card numbers and banking access passwords.
Absolutely do no open any such email. It's dangerous enough opening mail even from friends because their computer could be infected, sending the same infecting agent to you because it uses all the email addresses in their address book to send to.
