what is the purpose of giving order of qualifiers then?
The JSON still contains fields called "qualifiers-order" on each qualifier, as well as "snaks-order" on each reference. These are fragments from a feature the Wikibase software once supported. In 2013 the interface had "move up" and "move down" buttons that allowed to manually change the order of certain elements. This feature was not intuitive, bogus, actually broken, and removed just a few months later.
Much later it was replaced by an order that is consistent across all entities: https://phabricator.wikimedia.org/T99243 .
The "qualifiers-order" and "snaks-order" fields are unused and entirely meaningless now. They should not be used for anything.
The specific order you need for your application mostly depends on your application. If you want "preferred" statements to show up first, then go ahead and order them accordingly.
A concept of "importance" does not exist in the Wikibase software. Applying ranking is the the job of a (secondary) search engine, like Google does, or our very own CirrusSearch index. The only thing that can be considered a "ranking" is what you see on https://www.wikidata.org/wiki/MediaWiki:Wikibase-SortedProperties , which you can reuse if you want.
Best Thiemo