I'm reading about memory management in d3d12.  The docs basically say that a "committed resource" it like how past versions of Direct3D allocated a resource where you get virtual address and physical address.  I understand reserved resources to do things that tiled resources in d3d11 were used for, but when would you use placed resources over committed resources?

Placed Resources allow you to alias multiple resources over the top of another if saving memory is your goal. They also facilitate the approach of allocating a much larger amount of memory in a single go (a Heap) and then 'placing' multiple resources inside the same memory allocation. If a shadow map and a post-processing render target are not needed simultaneously in the frame then there's no need for them each to have their own memory.

As another example, you might choose to place an entire model's resources into a single heap. That might mean vertex buffers, index buffers and the model's textures have their sizes summed and a single heap is allocated of that size. Each individual resource is that 'placed' at the relevant offsets in that heap.