Well, ä is a character in German. You can either write it as LATIN SMALL LETTER A WITH DIAERESIS, or you can use COMBINING DIAERESIS and a. When you iterate over the German word Mädchen as Unicode code points you might be confused. Other languages do much crazier things.
How does utf8 make that difficult to answer?
When was the last time you iterated over a string of unicode points and said you know what would be handy right now? If these code points were split up into arbitrary and unusable bytes of memory.