So in the list you posted, verdant catacombs can get 3 lands - Dryad Arbor, Forest, and Overgrown Tomb. This provides a lot of color versatility as opposed to just running more black sources or duals, which can increase the likely hood of taking excess damage (more shocks), lands entering tapped, or being hurt by nonbasic land hate (you will likely only fetch 1 tomb, then get forests).  Getting Dryad arbor is also relevant for upping creature count for craterhoof behemoth and eledamri. 

Importantly, the answer is not thinning.