This reduces a body of text to the distinct words it contains — each one listed once, regardless of how many times it appeared.
Vocabulary rather than volume
A two-thousand-word article might use only six hundred distinct words. That smaller set is the article's actual vocabulary, and it is a far more useful thing to inspect than the full text when the question is what a page is about.
Common uses
Building a term list from competitor content, checking whether a draft ever uses the terminology its audience uses, or finding the unexpected words that reveal what a page really covers.
Pair it with frequency
Unique extraction tells you which words are present. A frequency count tells you which ones dominate. The two together are a rough but genuinely useful topic profile.
Try it: Extract Unique Words on SeoWolf's Notepad.