What it does seems just forcing the flow(building up color) effect, and the exact same effect is already achiveable with the current flow option...

Build up composites every brush dab individually instead of the stroke as a whole. You can see the difference on overlapping sections of the stroke, even more so if you use a special blending mode on the brush - like Addition and Color Dodge in this example: https://imgur.com/a/c3Or7ik
The Color Smudge brushes work in a similar way. One downside of Build up mode is that the Masked Brush functions aren't available with it.

Snudl wrote:Build up composites every brush dab individually instead of the stroke as a whole.

Yes, this seems already achievable with the current flow option. And you can even use masked brush with it.
So I don't see any use of the separate 'Build up' painting mode.

(Not that I want it to be removed. I'm just curious.)

No, again - the same thing is not achievable through the flow behavior, be it the old or the new style. Look at the example I linked and try it for yourself. Pick the Basic-4 Flow Opacity brush, then with very little pressure applied draw in the same spot over and over again without interrupting the stroke - it will only be as opaque as the maximum pressure you used. Change to the Build up mode and repeat - the stroke will keep on increasing its opacity as long as you drag the pen, even with very low pressure and even when using very low flow and opacity values.
Also compare the modes with a brush set to Addition blend mode and pick a bright color. Draw over the same spot multiple times without ending the stroke. In Wash mode the brush stroke is only applied as a whole to the layer contents through Addition, while each brush dab behaves like Normal blending. In Build up mode each brush dab uses Addition on top of the last one.

Snudl wrote:Pick the Basic-4 Flow Opacity brush, then with very little pressure applied draw in the same spot over and over again without interrupting the stroke - it will only be as opaque as the maximum pressure you used. Change to the Build up mode and repeat - the stroke will keep on increasing its opacity as long as you drag the pen, even with very low pressure and even when using very low flow and opacity values.

Is it? Looks like a brush with Wash + Flow curve behaves exactly the same as the brush with Build up + Opacity curve. Sure, just switching the Wash/Build up mode on a same brush would produce different results, but my point is that the effect is still achievable without the Build up mode option.

Am I missing something?

The most visible difference is when you use a blending mode. Fill a layer with black, select a color that isn't fully saturated, and set the brush blending mode to addition. You should now be able to clearly see the difference between buildup and wash on a given brush.