Warehouses
The warehouse objects (based on MooseObjectWarehouseBase) in MOOSE hold onto the objects that are built by the factory. They are templated so they can hold any of the types of objects the Factory can build.
The main purpose is to accelerate retrieval of sets of objects needed during assembly. To this end, they have many accessors on them such as getActiveObjects(). Internally, these sets of objects are cached to make retrieval quick.
MooseObjectWarehouse
MooseObjectWarehouse is based on MooseObjectWarehouseBase but adds important functionality for objects that support the SetupInterface. In addition, there is a bit of functionality there for holding sets of Kernels that go with each MooseVariable to make retrieving them quick for Jacobian calculations.
There is some trickiness in addObject(). To support getActiveVariableBlockObjects() we want to have a Warehouse for each variable Kernel's are acting on. To do that, we have nested MooseObjectWarehouses which we add to in addObject(). However, we need the recursion to stop after one level so we can pass recurse = false to keep it from recursing to death.