Many of the domestic subsidiary airlines were created because the local governments/investors wanted air service and were willing to invest the money.
JTA and RAC are partially owned by the Okinawa Prefecture government
is partially owned by 14 municipalities of Kagoshima Prefecture
HAC, which is no longer part of JAL, is partially owned by the Hokkaido Prefecture government
JAL Express (JEX) was created to lower costs in order to provide service to secondary cities in Japan. They have expanded to now fly some international routes to China.
J-Air was created originally due to the failure of another airline that provided commuter service in western Japan. They only fly regional jets.
JAA was no longer politically needed. More often than not you were on an aircraft that said JAL instead of JAA. In the end, there were only 2 747-300s that actually said Japan Asia Airways on the aircraft. Those were replaced with 747-400s wearing the regular JAL livery in 2004/2005.
JALways (JAZ) was a charter airline that was expanded to scheduled service on JAL's leisure routes. They tried to lower their costs by using contracted pilots from crew leasing companies and lower paid Thai flight attendants based in BKK
. They were main used on flights to Hawaii, Guam, Saipan, and Bangkok for most of their existence. They expanded to Australia then other countries in South East Asia in the mid-2000s.