Actually for Japanese in particular the extended CJK ranges are of limited use (I'm more familiar with Japanese than Chinese), although they do contain the part

Actually for Japanese in particular the extended CJK ranges are of limited use (I'm more familiar with Japanese than Chinese), although they do contain the parts of JIS X 0213 that were still missing from Unicode (all of them rare characters) and some ideographical component parts that are useful for dictionary and educational software. With the latter you can for example use Unicode codepoints for the distinct components of a single Chinese character (e.g., 𠂉 as the top-right part of 族). That is a fairly specialised use though. For Korean and Vietnamese the extra Chinese characters might apply to ancient names (geographical usually I would wager), but neither language has a lot of ideographs in modern usage. It's certainly an interesting topic though.