There's a lot that happens behind the scenes of a modern graphics API. For example, issuing a draw call may cause the driver to incur a flush of accumulated state changes, patch an existing shader, or even trigger a shader re-compile. Cutting down on what happens behind the scenes will allow developers to issue more draw calls within the same time and put more objects on the screen.
DirectX 9 had a similar problem of expensive draw calls but it was compounded by drivers living in kernel-space requiring an expensive context switch when draw calls were issued.
DirectX 9 had a similar problem of expensive draw calls but it was compounded by drivers living in kernel-space requiring an expensive context switch when draw calls were issued.