If you pre-validate the string because maybe you do a lot of transformations and operations with it later, then any functions processing strings become simpler. If you have the start of a 3byte character you can just read the next two bytes and don't need to check if any of the two trailing bytes is actually a nullchar instead (in case of null-terminated strings) or whether pos+2<len. Because otherwise you'd do an oob read. Also you don't need to check if the trailing bytes of your char actually start with 10xxxxxx, you can just mask off the first two bits right away.
