Information is the oxygen of the modern age. It seeps through the walls topped by barbed wire, it wafts across the electrified borders.--RONALD REAGAN
IF YOU HAVE A COMPUTER CONNECTED TO THE INTERNET, EXPECT THAT IT WILL BECOME A TARGET FOR A HACKER. While it's unlikely that hackers will specifically target your computer, that doesn't mean they won't leave your computer alone if they happen to find it while prowling around the Internet.
Hackers like easy targets, so even though they may not care about reading your letters to your grandmother or your 2002 tax returns, hackers may break into your computer just for fun to practice attacking a relatively safe computer, to use your hard disk as storage for illegally copied files (music, programs, or videos), or to plant a "zombie" program on your hard disk that can command your computer to flood a specific website with useless data in a denial-of-service attack. Your data may be unimportant to a hacker, but your computer itself can still be a valuable resource.
If you don't like the idea of someone taking control of your computer and having the power to wipe out your data at any moment, you need to protect your computer with a firewall.