1. To hide yourself from casual scanning users from finding your Mac on a network.
Originally Posted by Amie
2. Because most users want to share files on a network, especially in Active or Open Directories.
3. You can hide from most casual users on the local network.
Now when i get onto strange PUBLIC WiFi I use the simple NoobProof because it use the built-in command line ipfw and really hides my Intel Mac Book Pro from prying eyes. This ipfw has been refined for over thirty years so it is really robust.
Mac Pro Dual 2.8 Quad (2nd gen), 14G Ram, Two DVD-RW Drives, OS X 10.9.1
2006 Mac Book Pro 2.16 (first Gen) OS X 10.7.5
2TB Time Capsule, 2 TB
32G iPhone 4S Black, iPad (3rd Gen) 32G Black