To be fair, something is stepping it down from AC, and many modern PSUs step everything down from a common DC rail using DC-DC conversion (i.e, 12V, 5V, 3.3V, e

To be fair, something is stepping it down from AC, and many modern PSUs step everything down from a common DC rail using DC-DC conversion (i.e, 12V, 5V, 3.3V, etc are all generated from a source DC voltage). Then things like the CPU and GPU have long moved off of the 3.3V rails to harness the 12V rail for DC-DC conversion in various VRM stages, down to <<2V. My main appreciation for ATX12VO (12 volts only) is the MB can choose to right size the 5V and 3.3V rails appropriately. M.2 slots are driven from 3.3V only (even if M.2 devices step it up or down for their specific uses). A board with 4 M.2 slots will have a different potential 3.3V power demand than a board with only 2 slots. Typically, a decent board will have 1 M.2 2230 A or E slot for WiFi, and 1-2 M.2 2280/22110 M (and rarely B) slots for M.2 storage. PCIe x16 lanes will have 9W of 3.3V, as part of their 75W total power (the remaining 66W is 12V). As noted earlier, 5V is heavily I/O driven. A board with 15 USB ports can expect to need more 5V than a stingy board with only 8 (both are accounting for front I/O headers). I'd rather the MB have to right-size the 3.3V and 5V rails, than forcing every PSU to include copious amounts of 3.3 and 5 just to cover every edge case. Sure, it's a tiny bit of potential efficiency. It's also passing a bit of cost from the PSU to the board itself. Sadly, it does seem ATX12VO's main impact will be lost. OEMs are going to continue with their custom PSUs with unique connectors and the DIY vendor + consumer market has largely turned their backs to this.