The Capital Ships being sentient and armed spaceships, they can just enter planetary orbits and blast away till the planet is reduced to rubble.
It is shown that their Kinetic Barriers are incredibly powerful and it takes a lot of hits to damage them – which can't be landed because they are able to destroy the attackers quickly. So defenses will be ineffective…
Best Answer
They would require ground troops to search and harvest the planet's populace. They can not risk a species surviving with out a thorough search, for which ground based troops would be required.
Also once they have captured a planet, they would require adequate ground based infantry and cavalry to hold that position and defend against strike teams.