david weaver
They are intended to break the shaving off and prevent tearout on a western plane, it's clearly documented in literature from the early 1800s. It is so clear that it's even mentioned in an 1830s advertisement for thick irons suitable for back beveling that the double iron is a useful design to reduce tearout.

They wouldn't be on planes at extra cost when most of the users lived near starvation if they didn't do anything. There were already competent single iron planes available for less when they were introduced. If they didn't mitigate tearout, they never would've appeared.

It's extremely unlikely that they aren't intended to do the same thing on a japanese plane because they otherwise are not needed at all (they certainly aren't needed to add stability to the cutting edge, and adding a pin to a plane for no reason doesn't make a lot of sense, either).

Stu already alluded to the fact that they helped users plane subpar wood when choice materials weren't available.