This is just a "nice to have" feature; the ability to set the sampling quality per-map for the baker. I'm using an older/slower GPU so, to cut down on bake times, I use lower sampling for the maps that don't really need it.
Bake speed is not necessarily tied to sampling - with slow baking map types like AO, the amount of rays is really the important thing, and using higher sampling with AO maps can even bake slightly faster (with higher samples, we can use less rays). So if you're trying to speed up bakes, try setting the AO ray count to the minimum that you're comfortable. I find with dithering on, 256x rays looks very good in most cases. But you should do some tests.
What I like to do is set AO rays pretty low, to 64x or so, while I'm working. Then bump them up to 512 when I do the final bakes.
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What I like to do is set AO rays pretty low, to 64x or so, while I'm working. Then bump them up to 512 when I do the final bakes.