There is a less obvious benefit in more schedule-driven database architectures. By implication, the code knows exactly how much of which resources are instantaneously available at all times since it is directly managing those resources. This enables the database to dynamically adapt its behavior and scheduling decisions in response to specific resource pressures or excess resource capacity. This is basically a fine-grained form of internal micro-backpressure, dynamically prioritizing subsets of the workload that minimize impact on the resources under pressure at any moment in time. In some versions, you can switch internal execution strategies to take advantage of currently under-utilized resources. If you outsource this to the OS, you have no idea how close or far you are from the limits, it doesn't really work.
This doesn't add much performance per se but is great for minimizing tail latencies. It helps to create a robust internal resource equilibrium that handles transients and concurrency more smoothly and consistently than if the database couldn't make a fine-grained decisions about what to do from a position of total resource awareness.