Compulsory Dame Filling to Prevent Technical Sekis
In Japanese rules, eyes of groups in seki do not count as territory. This quirk of the Japanese rules could possibly be a holdover from the group tax effect of stone scoring rules, a historical forebear of the Japanese rules, (see this post in Pondering Upon The Rules Of Go - #17 by yebellz which also links to an article discussing the history of the rules of go), although certainty of its origin is likely lost to history.
However, this rule is loaded with complexity since one must define what is seki in order to distinguish whether eyes should count as territory. It can be very difficult to distinguish between seki and living groups. The concept is difficult to pin down with a definition that attempts to count eyes, false eyes, etc., since there all sorts of contrived positions like non-seki living groups the only appear to have false eyes, or complicated hanezekis and hanezeki-like positons. Ultimately, the Japanese rules simply define seki as living stones that are adjacent to dame:
Article 8. Territory
Empty points surrounded by the live stones of just one player are called “eye points.” Other empty points are called “dame.” Stones which are alive but possess dame are said to be in “seki.” Eye points surrounded by stones that are alive but not in seki are called “territory,” each eye point counting as one point of territory.
This rule has the side effect of compelling players to fill nearly all dame (except those that must be left unfilled to preserve actual sekis), since otherwise any non-seki living stones that are adjacent those dame points would be considered seki and have their territory nullified.
It’s common practice for players to leave dame unfilled when playing with Japanese rules. This is a result of the common misconception that dame do not have to be filled under Japanese rules, however, as we see from the discussion above, the opposite is technically the case! I’ve previously discussed this discrepancy between this common practice and the actual Japanese rules here: Japanese Rules Popularity - #56 by yebellz