If your laptop is stolen, it is relatively quick and simple to access data stored on its hard drive, including web browsing history, patient information, legal documents, personal information, etc.  Using open-source (free and community developed/supported) software to encrypt your laptop drive raises the "security fence" high enough that only super-spy types could access your data.  And if they have your laptop, you have more to worry about than your college videos ending up on YouTube.

System drive encryption, which can be done on laptops, desktops (home and office), and servers, is fairly simple for your IT guru to perform, and greatly increases the security of your personal data.  The process encrypts every bit (literally) of information on the drive, such that a key is required to decrypt it.  On full system drive encryption, the key is entered once upon booting.  Only you know the key, so all of your information is safe, even if the drive is removed and analyzed on another system.

Further Reading:

An interesting article from 2008 regarding laptop theft in airports

A Wikipedia article on Full Disk Encryption: