They often have embedded php 'images', too small to see. When you load the email, you call the php page with your email as a variable. This shows that the email address is active, and that the person who owns the email opens stuff from random people. This makes the address very valuable. They can then send their first waves of real spam to that address knowing that it will have the maximum effect at good addresses before it starts getting filtered. It also makes their lists more valuable for sale.

This is why places like google block images by default -- they can be used against you.

But that is only some of them. I'm pretty sure I've gotten those random emails without an embedded image too. No idea what the purpose is. To check if it bounces?