This tool tests for the JSON mappings in the go-github data types. It will identify fields that are returned by the live GitHub API, but that are not currently mapped into a struct field of the relevant go-github type. This helps to ensure that all relevant data returned by the API is being made accessible, particularly new fields that are periodically (and sometimes quietly) added to the API over time.
These tests simply aid in identifying which fields aren't being mapped; it is not necessarily true that every one of them should always be mapped. Some fields may be undocumented for a reason, either because they aren't actually used yet or should not be relied upon.