Native Instruments PXI backplanes are made for scientific instruments so you throw on a couple of mobo modules with modern intel CPUs hooked up through the PCI

Native Instruments PXI backplanes are made for scientific instruments so you throw on a couple of mobo modules with modern intel CPUs hooked up through the PCI backplane lanes to RF generators, motor controllers, sensors and diagnostic equipment like spectrometers, network analyzers, etc. The backplane comes with more specialized power sources and a bunch of other stuff too. If you need, for example, a network analyzer hooked up to an SDR that can live decode and compare tens of gigabits of sensor data to the output for validation and tuning you might need several CPUs ingesting and processing that data talking to another CPU that actually runs the SDR software and so on. For these kinds of scenarios, a few PCIe cards and a rack really isn't enough (or cheaply available) compared to a PXI backplane.