How does local dimming affect SDR content?

[betonKruglosuTotchno]

I did not care about local dimming until recently but there's TCL 32R84 which seems to be great on paper with 1400 zones and 1400+ cd/m2 and most affordable 32" 4K FALD monitor.
There's also a chinese rebrand Ffalcon R32U8 which might be even cheaper.

Hence the question: how does local dimming affect SDR content with no dark tones? Is it used to save power? Does it change the image in any way? Rtings seems to only have text description of local dimming algorithm.