It's not essential at all, it's for these cases where a text containing these extended glyphs which include japenese characters is about to be displayed, since the default font does not contain any japenese character, it's a good idea to have a fallback...
There are sometimes japenese characters in the history.dat file, and I also just finally found a safe way to convert these old japense long names that some drivers had to utf8, so they are now displayed in the "driver info" you can access from the "About..." dialog.
And you can get japenese characters in the file selector if some rom dumps or neocd images filenames contain japense characters of course ! About these long japenese names in some drivers : even Antiriad forgot where they come from and how they were maintained ! They were displayed until now only when using the -gameinfo or -li command line switch, so on the console, and since most of these japense names hadn't been converted to utf8, it didn't display anything usable ! Now afaik they have all been converted to utf8, so they display correctly everywhere assuming you have configured a font for that.
Nothing much to say about it, by default there is no japenese font, so you'll see probably empty squares instead of the chars, that's what is used the most frequently. A good selection in linux is the droid font, DroidSansFallback.ttf works well.
In windows just take a very big font, malgun.ttf which is distributed as standard with windows 10 works well.
If you select as japenese font a font which doesn't contain these characters, it's like doing nothing of course.
There are high level APIs for that which automatically select a font installed on the system which contains the missing chars, but it would be overkill for raine, so you have to select the font manually at least once, it's better than nothing and we avoid 1 more dependency !