In this case, the codepoints were added in part because the proposers could show many printed works (user manuals, I guess) that included sentences such as "to turn the foobar on, press the ■ button", which shows that the glyph between "the" and "button" is in some way like the surrounding glyphs. Chessmen were added for similar reasons, even though very few people actually read either user manuals or chess literature.
In this case, the codepoints were added in part because the proposers could show many printed works (user manuals, I guess) that included sentences such as "to turn the foobar on, press the ■ button", which shows that the glyph between "the" and "button" is in some way like the surrounding glyphs. Chessmen were added for similar reasons, even though very few people actually read either user manuals or chess literature.