When I hear "slave mode", I think optical slave, which is just a dumb light sensor that triggers a flash or studio strobe to fire at a manually pre-set power setting. Just for clarifictaion, the SB600 does not have optical slave mode, but it does have slave mode for the Nikon Creative Lighting System (CLS). Nikon's CLS uses an infrared signal from either another hotshoe flash unit with commander mode, a pop-up flash of a camera with commander mode, or a dedicated hotshoe-mountable commander unit. Infrared requires line-of-sight, or at the very least the slave unit's sensor needs to detect some bounced signal from the commander unit. Sometimes, in bright sunlight outdoors CLS isn't reliable, as the ambient light overpowers the infrared signal.
Radio triggers don't have the same limitations of infrared. Radio triggers can work through most walls, outside in bright light, and over great distance. Some triggers, like optical triggers and PocketWizards, only send the "fire" signal to the flash. Others, like CLS, RadioPoppers, and the new PocketWizard Mini and Flex units, can transmit TTL or manual power settings to the flash.
If I'm indoors or don't need line-of-sight, I'll use CLS with the pop-up flash of my D700 or D300. I use RadioPoppers to adjust the manual power settings of various off-camera flash all from my camera, especially when they don't all have line-of-sight. Sometimes CLS works with a flash inside of a softbox, but not always. So, I usually use the RadioPoppers whenever I use a softbox, just cause I'd rather not deal with misfires. Some of my old flash units do have optical slave built-in, and if I know what manual power setting I'll use for it, and it won't be mounted in a hard-to-reach place, I'll just use its optical slave mode without a RadioPopper receiver.
