The most important feature of secondary sources is that they offer an interpretation of information gathered from primary sources. A tertiary source presents summaries or condensed versions of materials, usually with references back to the primary and/or secondary sources.
Being able to categorize a source helps you understand the kind of information it contains, which is a big clue to (1) whether might meet one or more of your information needs and (2) where to look for it and similar sources.
