Drivers for older APIs have needed to be complex in order to maintain performance on legacy applications. Different optimization choices have led to unpredictable performance variations between systems; this means that application software may run perfectly well on one system but inefficiently on another. The complexity also means that drivers can contain bugs, or that features which are intended to help the behavior of one application can have a negative impact on another.
Vulkan will make it easier for application authors to ensure good performance across a wide range of devices. In addition to improving the overall performance of graphics software, this consistency should lead to fewer bugs and more flexibility of different types of hardware.