Why does inventory.data.gov exist?

I’ve just started exploring the world of open data and towards that aim I’ve been looking the data sets on catalog.data.gov and inventory.data.gov.

What I’m struggling to understand is the purpose of inventory.data.gov. The about page states:
“Inventory.data.gov is a tool for federal agencies to create and publish metadata catalogs”

Isn’t the metadata for each data set published on catalog.data.gov as well? If so, then what purpose does the inventory.data.gov site serve?