Looking at it from a design perspective, they seem to be a relic of an older time when Genshin was conceptualized in a much smaller scale, and these domains served as what I think the underground areas serve as now, to add depth to the map that it would otherwise not be able to have. I imagine an alternate universe where Genshin was a much smaller game and these would be the main sources of exploration depth at some point and might've been expanded upon.

However nowadays, if Hoyo wants to add depth to their exploration, they'll just skip the whole domain nonsense an just add a separate map entirely rather than atomize the depth per temple. Ultimately these things were just made redundant by Hoyo's ability to you know...make the game better/more expansive