I think the issue is:
Unless otherwise specified, and without explicit synchronization, the various commands submitted to a queue via command buffers may execute in arbitrary order relative to each other, and/or concurrently. Also, the memory side effects of those commands may not be directly visible to other commands without explicit memory dependencies. This is true within a command buffer, and across command buffers submitted to a given queue.
The barrier is not needed if a semaphore is submitted after a command buffer. We could track that and only emit the barrier if necessary.
The "Unless otherwise specified" there is important though; as mentioned earlier, the Vulkan spec does in fact specify an ordering for a lot of operations. The linked MSDN text is quite a bit less precise, but at first sight it doesn't appear to give any additional guarantees about availability or visibility of the results of operations; I imagine the number of affected operations to be fairly limited.
To stick with Giovanni's earlier example, suppose that in Vulkan we have a command buffer A containing a render pass with a vkCmdClearAttachments() call that clears a particular render target to red, and a command buffer B containing a similar render pass clearing that same render target to green. If we submit those with a single vkQueueSubmit() in the order {A, B}, the render target colour will end up being green. Specifically, submission order is specified to be {A, B} in that case, primitive order doesn't allow reordering them, and rasterisation order specifies that the resulting writes happen in primitive order. That's generally true for other drawing commands as well. On the other hand, transfer operations (AFAIK) don't generally have such ordering relative to each other, or relative to draw commands, but they do generally require layout transitions in order to switch between using them as transfer source, transfer destination, or with draw commands. The question really ends up being which guarantees d3d12 provides that we don't already get from Vulkan.