The spore ships are unique to the Korath. Think of them as troops transports, except without the troops and a lot more dangerous. They can capture world in a similiar many to troop transports, but they don't invade. They instead pump toxic gases into the atmosphere killing all inhabitants without a fight, turning the world into a toxic class. They then plant a Korath flag once done, making the planet yours, and then jump back into their spore ship to do the same to another.
Incase you didn't get that, let me summerize:
-Can capture worlds.
-Automatic success.
-Turns planet into toxic class.
-Spore ship is not used up in the process.
Its a powerful super ability.