It will be used with Writs once they are fully in play. You need to have a minimum of 500 prestige to get the writs (as well as 150 skill). I believe that getting writs will also lower your prestige, requiring you refresh it to get more writs -- but I may be mistaken in that regard.
Prestige is diplomatic currency. Sometimes you flash it to open a door to more jobs - that's how Writs work.
Sometimes you spend it to get things done. Sometimes you can buy cool items with them. Little of this is in the game at this time, but you will see more soon, starting with the House factions in New Targonor.
You won't have to spend prestige for Writs. You just need to be at the appropriate prestige level (500) with the city where you want to obtain the Writ.