Understanding local dimming zones

[Things change I've changed]

I'm researching older 38" gaming monitors, for example I'm temped by this,

LG 38" 38GN950P-B 3840x1600 NANO IPS FreeSync G-Sync Compatible 160Hz 1ms HDR 600 Curved Gaming Monit

Order LG 38" 38GN950P-B 3840x1600 NANO IPS FreeSync G-Sync Compatible 160Hz 1ms HDR 600 Curved Gaming Monit now online and benefit from fast delivery.

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The screen only has 12 dimming zones, looking at videos of this online if example there is a bright object in the corner of the screen with a black background that whole area around the object lights up which looks very bad, what I don't get is what is this achieving?, the reason I say this is if I have that same scenario on my current old X34 with no local dimming zone I don't suffer this problem and the screen looks to my eyes 'fairy' black around the bright object with no light bleed? It makes no sense to me, can this feature be turned off, and if it can I assume it would then act similarly to my current screen?

 

[Things change I've changed]

It's an IPS panel, I'm just struggling to see the benefits, look at this image for example,

You see where the screen is lit up up the left where the mouse is, are you saying with local dimming disabled the whole screen would be that grey? I have never witnessed any monitor without local dimming zones to be that poor at blacks, or is this due to the HDR brightness?