That project is not an encoder/decoder - it just returns values of keys. They are not comparable as the use cases are entirely different.
But often that struct isn't your end goal in life, such as when your data is being transformed, dispatched on, or being moved into another yet data structure. In that case unmarshalling to an intermediate structure, while convenient, is also extra computational work.
Buger's parser isn't the first thing I'd reach for, but if JSON parsing performance is an issue, ObjectEach() can be used in many places where you'd otherwise invoke json.Unmarshal.