It’s usually an excuse for basically “I like this Korean fc but I want my character to be Japanese/Chinese/Thai”. But like, imo, they could make their character born and raised in those countries if they want to but just keep their ethnicity?? Or, yeah, just make them Korean lol. It usually applies to canon characters, I mean. Like… using Korean fcs for Japanese cartoon/comic characters etc.
& I wouldn’t say that certain “features” are specific to different Asian ethnicities… without fashion, makeup and other beauty trends, it’s not easy to tell people’s ethnicities apart. At least, from my experience. Although people pretend to know, lol. My brother’s girlfriend always gets asked if she’s Filipino. But that’s usually just some colourism for you.