There are many reasons why someone would want to use a keylogger, in some cases parents want to 
know whats up with their kids, in some other cases, script kiddies use them to steal private information 
that the victims type on their keyboards, including usernames and passwords of any kind of service.

Keylogging is a very old sport and has been around since the first trojans appear in public after 1997.

There are many ways for a script kiddie to infect dozens of people with a keylogger, by sharing an infected 
file with the keylogger in forums, public sites, chatrooms etc

Once the script kiddie has enough infected victims, he starts gathering valuable information, and will 
slowly build a small database of information of each victim, some of the infected computers will be 
shared by more than 1 people giving even more to the script kiddie.

By the end of those kind of attacks, a script kiddie can gather:

email accounts (which include even more information)
paypal accounts
egold accounts
credit cards
online banking accounts
game accounts

any kind of password that can come in your mind.

Then, a script kiddie would use forums and IRC channels to openly trade his goodies with other kinds of 
stolen information, he would trade a credit card for a paypal account for example, in some cases script 
kiddies sell the stolen goodies and make alot of money!

Remember that this is extremely dangerous area which script kiddies often trade with unknown 
individuals from around the world that themselfs are doing other, unknown to the script kiddie illegal 
stuff. No matter how careful the script kiddie was with the steps he took, he might get busted at the 
end because of someone else.

The chatrooms with the illegal trade are usually located in UNDERNET IRC servers.