See Joel spolsky's article "The Absolute Minimum Every Software Developer Absolutely, Positively Must Know About Unicode and Character Sets (No Excuses!)".
To quote the part that answers this questions concisely
The Single Most Important Fact About Encodings
If you completely forget everything I just explained, please remember
one extremely important fact. It does not make sense to have a string
without knowing what encoding it uses. You can no longer stick your
head in the sand and pretend that "plain" text is ASCII.
This is why you must tell ruby what encoding is used in your file. Since the encoding is not marked in some sort of metadata associated with your file, some software assumed ASCII until it knows better. Ruby 1.9 probably does so until your comment when it will stop, and restart reading the file now decoding it as utf-8.
Obviously, if you used some other Unicode encoding or some more local encoding for your ruby file, you would need to change the comment to indicate the correct encoding.