Orca is a stealthy assault submarine. They are equipped with powerful torpedo launchers that can deal some lethal damage to structures and ships within seconds, though they are also very vulnerable due to low health. They are kind of a glass cannon.
Orcas are invisible to radars and the units on the surface, so if your enemy is not careful enough you can sneak one or two of these into his base and cause some havoc.
The main use case for those is taking down underwater metal extractors and trapping fleets, which don’t include proper anti-submarine units, like Destroyers.