The answer to this question varies. Often they are used to take over control of your PC or laptop computer. The reason is simple: money. When they have control of your PC, they can use it to host illegal websites and illegal content such as prescription drug sites where they steal credit card information and identities. If your computer is infected, get it repaired so you aren’t helping steal identities! Also your computer can be combined with hundreds of thousands of other computers they control, called a bot-net, which can take down a website in a DDOS attack in order to extort money from the website owner or also because the website made them mad (such as an anti-spam website). They can use their bot-net to break into secured data that would otherwise be impossible for them to break. They can your computer to send spam which is primarily used to try and get people to buy illegal items and all they do is can steal the credit card information and identity of the person purchasing the item. And what’s most interesting is that much of this comes out of Russia and China!