You use SunScreen in Stealth Mode if you do not need routing functions or if you want to decrease possibilities for attacks by hiding the topology of your network.
SunScreen Stealth Mode acts much like a bridge--no IP interfaces are exposed to the public or private network and packets are transparently passed through the Screen. This mode cannot be directly attacked through any means other than a denial of service attack, and cannot be seen or detected through traceroute or similar network tools.