The only difference is that the Engine in System 8 has three host ports, while the SSP only has two.
On the Engine, the third host port is the same port as the device port, and automatically switches to device port role when you plug the device port into a mac or PC. So in that case the third host port is disconnected. When there is no PC or Mac connected you have a third host port you can connect devices to.
The USB host ports can be used for MIDI controllers or a USB ethernet adapter (if you want to develop software for the SSP or Engine).
The USB device port is used for streaming audio to your Mac or PC.