I’m referring to a server without a dedicated boot disk. This is not especially rare.
On an edge-style machine, you generally have a very limited number of M.2 slots, and dedicating one to a boot device uses up potentially 100% of your potential storage capacity, not to mention that an extra drive would be a non negligible fraction of total cost.
On modern servers, there may not be a usable SATA controller, and NVMe usually costs 4 PCIe lanes. Those aren’t free. You also end up paying the absurd OEM premium for an extra disk if you go with a big name OEM. Sadly, there is no industry standard cheap, reliable boot device standard, at least as far as I’ve ever seen. Maybe someone should push USB3 for this use case — the price is certainly right, and performance is likely just fine.