The most important reason is semantics. If "O" and "0" look alike in a certain font, should we use the same character code for both? No, because they have diffe

The most important reason is semantics. If "O" and "0" look alike in a certain font, should we use the same character code for both? No, because they have different meaning. Here are some contexts in which this semantic difference is important: search (compare search results for "cop" and "сор"), alphabetical sorting, text-to-speech, spellchecking, case conversion ("ATOM" -> "atom", but "АТОМ" -> "атом", note the difference between t-т and m-м).