Google has a dedicated ranking mechanism for long articles
Related content outcomes more visits
For long articles, Google gives improvement in ranking
Support visitors on website longer, increases appointment
Structuring content provides more spending to website

Well, I totally agree with you that descriptive article is most preferable thing for Google but it must be unique and informative for the visitors.

Earlier it was said that you can convey the information in 400 words but now the users demand complete and step-by-step answer of the particular question and so the evolvement of long form content came into existence.