It's not a Microsoft invention, and MS's use of it is really quite sensible. They had a problem of distinguishing UTF-16, UTF-8 and non-Unicode (possibly a single-byte "extended ASCII" type encoding, possibly some multi-byte monstrosity) text files. Since UTF-8 and ASCII-compatible encodings look similar when there aren't many >U+007F characters in use, and identical if none are in use, they could get confused. Prepending a Byte Order Mark solves this problem, in that it makes a file unambiguously UTF-8 (or UTF-16, for that matter).