The reason why they had to capture the Poison Fog Earthworms was that their attack was just a precursor; more importantly, Su Han saw in them an excellent potential for becoming "Black Slaves."
Putting aside their ability to create poison fog, their sheer size and remarkable burrowing capability alone made them excellent construction units.
If combined with the Black Rock Tree Monsters and Tree-born Worms, the underground construction capabilities would greatly increase.
The idea of constructing underground connections between bases had come up before, and underground tunnels were a post-apocalyptic version of an answer.
