SKs are no longer ships.
You can destroy ennemy structures by having a percentage (from 0 to 100) of your pods to release their captured roids onto the target planet (hello Starship Troopers). Precision is not perfect not all the roids will hit a building.
To stop roids hurled at your planet, you can have defence to send pods, (in addition to def ships) or you need some pods in a fleet slot at home (not at the base fleet). Each pod has a small chance of recapturing the roids.
That chance can be increased by building the Early Warning System structures...
