Anyone explain what they do and why?

no one?

You mean a tapered steerer?

I think they are supposedly stronger and save weight, but it seems like another marketing ploy to me. MBUK was saying recently that standard steerers will be 'obsolete' soon. 🙄

On these, is the actual steerer tapered, or is it just stepped at the bottom?

How does a headset take into account the angle the bearings need to be at for the taper given that head tubes are different lengths?

I got challenged on the last similar thread for pointing out that (most) 'tapered' steerers arent really tapered but step down a short distance into the head tube. The larger lower bearing may be beneficial,as is a larger headtube, but the small amount of tapering to the steerer isn't doing much.
My opinion, of course.

The force on the bottom is greater than the force on the top, so the concept is bang on… Whether it actually makes the slightest bit of difference, I don't know. It seems like the big headtube offers construction advantages for bikes though, not to mention having space for headbadges the size of a knight's shield.