Sandboxing your app is a great way to protect systems and users by limiting the resources apps can access and making it more difficult for malicious software to compromise users' systems.
As of March 1, 2012 all apps submitted to the Mac App Store must implement sandboxing. Enabling the default sandbox environment is as simple as checking the Enable Entitlements checkbox in Xcode target settings, allowing you to begin sandboxing your app.
If your app requires access to sandboxed system resources you will need to include justification for using those entitlements as part of the submission to the Mac App Store. Apps that are being re-engineered to be sandbox compatible may request additional temporary entitlements. These entitlements are granted on a short-term basis and will be phased out over time.