Here are some reasons:
Summative annotations (also known as "informative" annotations) provide only a summary of the author's main ideas. Summative annotations are typically two to three sentences long and include no statements of the source’s relevance to your paper or critical remarks evaluating the source’s quality.
Summative annotations may include the following type of information:
Evaluative annotations (also known as "critical" annotations) summarize the essential ideas in a document and provide judgments—negative, positive, or both—about their quality. Evaluative annotations are typically three to four sentences long. Evaluative annotations usually begin with broad comments about the focus of the source then moves to more details. Your comments should move from the details of the text to your evaluation of the source.
Evaluative annotations may contain the following type of information:
What is an Annotation?
An annotation summarizes the essential ideas contained in a document, reporting the author's thesis and main points as well as how they relate to your own ideas or thesis. There are two types of annotations: summative (aka informative) and evaluative (aka critical) - see examples under the 'Types of Annotations'. Annotations are typically brief (one paragraph)
Briefly examine and review the actual items
Choose those items that are most relevant. You may wish to provide a variety of perspectives on your topic.
Next, for each item: