Spam used to be the domain of small time scammers and used to sell everything from university degrees to hair restoration potions to creams supposed to enlarge certain parts of the anatomy. These spams were relatively few in numder, didn't take up too much room in inboxes or place an excessive burden on email servers and were usually relatively easy to block. But times have changed. Today, while small time scammers are still responsible for some of those spams, the majority are the work of organized criminals. And they being are sent out via botnets in unprecedented numbers.
The profits of spamming can be enormous. Complex pump-and-dump scams in which spammed "hot stock tips" are used to artificially inflate share prices can net the perpetrators millions of bucks (example) and phishing scams can provide criminals with access to a mass of cardholder data and sensitive corporate information. Gartner estimate that $3.2 billion was lost to phishing scams in 2007.
The fact that so much money is at stake has driven the spammers to constantly hunt for new ways to get the emails past spam filters and to make their scams more appear more convincing (take SonicWALL's phishing test and you'll see how difficult some phishing scams can be to spot). And it has also served to drive an increase in the volume of spam - approximately 75 billion junk email messages are sent out each and every day.