What is Inventory for?
Applying your prices and rules to actual dates. Rooms & Rate Plans is where you decide what your rates are; Inventory is where you say when each one applies.
Its own subtitle sums it up: "Manage rates, availability and restrictions across all room types."
This is where a season becomes real. You built a high-season rate group in Rooms & Rate Plans; here you drag it across December and January and it is live on every channel — usually within a minute, with nothing to press.