The reason why is that JST output is stereo amplified and not line level. If you connect it to anything line level it will in turn get amplified AGAIN resulting in either your amplifier on your board dying or amp on motherboard dying from overheating.
Usually when people connect amplified audio to something like an amplifier they use a high to low converter.
This 200% and yet I see people neglecting it on so many occasions and just wire straight RCA plugs to this stereo header.There is even some jackass on Ebay selling premade adapters from the JST XH to RCA ffs...
same for several other boards like Neogeo MVS btw... or Konami boards with stereo headers
General consensus if you do something stupid like that is to keep the volume low on the arcade board if sending to a amp/line level device. Less chance of blowing out your amp since the initial signal isn't fully erect. Turn it up at the device, not the other way around.
Seems dumb but it is technically more safe. Best way is have the knowledge that a high to low converter is cheap and makes this so much safer. Much better than replacing the amp on your receiver/board + other stuff when it gets done having a battle of amplification.