Cost:
Enterprises spend an average of $130,000 a month in IT time fighting spyware related issues.(Source:NewDiligence) Dell estimates that twelve percent of its support calls involve spyware issues. And that doesn't even take into account the lost time of the employee facing the issue. If you're an individual or a corporation, you need to know about spyware because it costs you time and money. Who wants to spend money fighting this stuff when they could be making profits? And who wants to be cleaning up their computer when they could be lazing in the sun at home? I'm sure you don't. Cost should make you want to know about spyware. But even if it doesn't, the security issues should.

Security:
It is estimated that ninety percent of all PCs have thirty or more spyware programs running on them. That will slow down your machine for sure, but it could do more insidious harm. Spyware is increasingly being used by criminals. And their favorite type of theft is credit cards. This allows them to start purchasing on your name and reputation. And this type of activity isn't just the domain of the petty thief. Organized crime is now investing big dollars in getting at your bank accounts and credit cards.

Privacy:
Closely related to the above topic, spyware invades your privacy. Folks are intentionally trying to see where you go on the internet, who you send emails to, what you enter into forms, what documents you look at, etc. They try to collect information on who you are, what you like, and where you spend your money.

Performance:
This really is a no-brainer to anyone who has had spyware on their machine. Spyware can utterly destroy a computer. I have seen machines that no longer boot because of it. Network connections can be trashed. You don't want this headache.

The best medicine is prevention.