Prestige is as useful as Gold and Silver. This is something that one cannot become strong without.
Developing a crew member
Upgrading a crew member
Upgrading both Character Aptitudes and whole crew Aptitudes
How to Obtain
Fight in Impel Down. (a very good source)
Fight in Dungeons.
Fight in Elite Campaigns.
Do Partner Battle Royal and collect rewards.
Save prestige from the start of your adventure. They will be very needed in the later stages. Developing one (1) crew member to his/her full potential may take 50 million Prestige depending on luck.
Stay tuned for more updates to the Prestige System.