Typically the application wants to draw stuff on the GPU. To do this it calls into the GPU driver, which builds a big memory buffer in some implementation-defined format that instructs the GPU what to do, and then the driver instructs the GPU to execute that.

However, applications these days want to do some of the work to determine which commands to do on the GPU (e.g. culling).

So the work here is to allow the application to create a memory buffer on the GPU with commands using a standardized format and then the driver has to convert that into the implementation-defined format on the GPU. This copies a lot of the complexity of the driver from the CPU to something running on the GPU.