Here's why:
* Commas are used to separate elements in a series or to set off nonessential phrases.
* Definition or restatement clues often take the form of a phrase or clause that directly explains the meaning of an unfamiliar word.
Example:
"The avifauna, or birdlife, of the Amazon rainforest is incredibly diverse."
In this sentence, the phrase "or birdlife" acts as a definition clue for the word "avifauna." The comma after "avifauna" separates it from the restatement, making it clear that the phrase clarifies the meaning of the unknown word.
Other types of context clues, such as example, contrast, and inference, may not always be offset by commas.