FileVault 2 can make nations quake, apparently, but it’s just a bit of good information hygiene, letting you make choices about the degree of vulnerability you want to tolerate for your locally stored data and any software or stored passwords for services in your accounts. With it off, you’re not risking everything, but with it on, you have a high degree of assurance about who can access what.
My son’s MacBook Air got stolen last year when his apartment was burglarized. We spent a lot of time together changing passwords. With File Vault, we wouldn’t have had to do that. I strongly recommend using File Vault.
How to encrypt your Mac with FileVault 2, and why you absolutely should | Macworld.