As ronaldh said. (though I would choose ASA over classic in most circumstances)
the key points to note about JASAs (not to mention YASAs and FASAs) is that they are award seats that earn FF points and status credits. They almost always 'cost' more than classic awards, but the taxes and surcharges are included. You can also 'wind back' the point amount required which simultaneously 'ups' the dollar component.
an example: A classic one-way J ticket MEL-PER is 36K points plus $$$ taxes and surcharges (which may also be paid with points if desired). no points/SC earning.
a 'well-priced' JASA MEL-PER may cost say 46K points, with nothing else to pay. It will earn full FF points and SCs at business class levels. However if you have only 38K points available you can wind back to 38K, and it will add a dollar amount to pay as well. Still full earning.
Points 'prices' of JASAs roughly equate to the $$$ value of revenue seats left for sale...usually the closer the flight the more 'expensive' they are (there are some ridiculous ASAs available out there for amounts in excess of one million points)
They are seen as potentially very useful tools to assist status retention.