This is a question that I'm frequently asked and the answer depends on the type of spam emails that are being sent.  Most spam emails can be categorized into the following four groups:

1. Product/Service Spam

These are emails that are trying to sell you a product or service.  More often than not, this will be a magic pill to increase certain parts of your anatomy!  The Product/Service Spam group is the most common form of email spam.  They include spam emails that link through to websites, as well as direct sales messages.

The reason for spam:  

The reason Product/Service Spam emails are sent is simple... to make money.  The spammers are able to send out millions of spam emails every single day, with minimal costs.  It only takes a tiny fraction of a percentage of the recipients to click through and purchase one of their products for the spammers to make some serious money! 

2. Pump and Dump Scams

A "Pump and Dump" scam is basically an email that tells you of a great stock/share that is incredibly cheap at the moment, and that you should rush out and buy the stock/share.  The scammer would of already bought a large number of the stocks/shares before sending out the spam email.

When lots of people then start buying the stock/share the price will quickly rise.  When the price reaches a certain level the scammer will dump (Sell) his stocks/shares for a large profit.  Usually the companies involved are failing companies with no real value so when the scam is realised the price crashed leaving the other people  who bought the stock/shares with massive losses - but the scammer has already cashed out and made his money well before this happens! 

The reason for spam:

Again, the reason is simply money.... As long as enough people believe the scam and buy the stocks/shares the price will rise rapidly allowing the scammer to dump his shares before the price crashes. 

3. Fraud & Phishing Emails

Phishing emails are the emails that look like they have come from your bank (Or any other online site where your personal details are stored), asking you to logon or confirm your account details.  A common Phishing email may be forged email asking you to confirm your account logon details for your online banking.  The emails and websites used often look identical to the real sites but will be used by criminal gangs to capture your logon details which can then be sold on to other criminals or used to withdraw or transfer money from your online accounts.

Fraud emails will usually either ask you to help with the transfer of a large amount of money from a failing African country, or will be informing you of a large lottery win.  The catch with all of these scams is that the fraudster will be asking for some money up-front to help with the transfer of the money to you.  

The reason for spam:

These types of spam emails are sent out by criminal gangs, often part of larger organised crime gangs who may be involved in weapons trading, drugs and illegal pornography.  The Phishing emails have a simple aim - to get as many logon details as possible.  These logon details will then either be sold on to other criminals or used directly to defraud money from the victims... usually by simply transferring funds from the online accounts.

The fraud emails are often run from African criminal gangs, the most famous type is the Nigerian 419 scams - where the fraudsters will be asking the victim for help transferring millions of dollars out of their country.  They will offer the victim a large percentage of the money in return for their help.  The catch is that they persuade the victim to put up thousands of dollars to help with the transfer.  Needless to say the large some of money never materialises once the victim has paid the money.

4. Virus/Trojan Emails

Many spam emails are sent out through infected computers.  These infected computers are controlled by criminal gangs to send out spam for other fraudsters, and are often termed a "Bot-net"... a collection of hundreds or thousands of infected computers that are under the control of the criminal gangs.

The criminal gangs use viruses and Trojans sent through spam emails to infect more computers... the more computers they can infect, the more they can control and then hire out to fraudsters and other spammers.

 The reason for spam:

Spammers needs computers to send out their spam emails, the more computers they control the more spam they can send out.  Often they will rent out time on the computers they control (Bot-Nets) to other spammers.  To control more computers the spammers will send out virus emails that infect the computers and allow the spammer to control them to send out spam.