Maybe your source was written by a scholar, maybe your source was written for a fifth grade music project. Which one of these is likely to be more accurate and more trusted by your professor? That's why you need evaluation.
Check that your sources are valid the easy way by answering these questions:
WHO: Who wrote/created this source?
WHAT: What are the author's motivations in creating this source? Are they academic or opinionated?
WHERE: Where are the cited sources in the bibliography from? Do they appear reliable?
WHEN: When did they create this source? Have there been advances since then?
WHY: Why is the author qualified discuss this subject?
HOW: How did you find this souce? Did you use a database?
Peer Reviewed sources are also generally reliable, but you should always make your own intelligent evaluation of a source before you take it as fact!