20+4-pin EATXPWR and 4+4-pin EATX12V (and/or 8-pin EPS12V) are dedicated PSU power outputs.
ATX12V (and EPS12V) PSU standards require these 12V power outputs have dedicated regulation.
These single-purpose outputs should not be combined with other 12V output rails. Although of course you can buy adapters which will plug 6+2-pin PCIE or 4-pin Molex outputs to EATX12V/EPS12V. And many people do use these without any issue, at least on robust PSUs.
But I think these are just asking for problems. Not just error messages and failed boots but also system instability, throttled performance under heavy load, weak overclocks, even electrical damage to the motherboard (and anything plugged into it). I seriously believe the only smart option is to always buy a PSU which is already properly equipped with all the needed power outputs for the platform hardware.
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