Side Missions Overview
Side Missions are the Mafia: III equivalent of grinding- you run the same mission over and over again to improve your stats. While you can run side missions for any of your underbosses, the format is generally the same for each - drive all the way out to the back end of the bayou - overcome armed opposition to steal something in a vehicle, and then bring that vehicle (which will invariably be slower and more cumbersome than whatever you drove out) to one of the rackets owned by the given underboss.
This improves that racket's earn, which in turn gives you more kickback money, and can unlock bonuses from that underboss. You also get a whisper of back story about each underboss' lieutenant - Alma, Emanuel, and Nicki. Important Notes - You can only run a side mission once for each racket a boss owns.
The game screen will tell you when all rackets for a boss are at maximum earn, but the characters will continue to act as if you need to go out and do more missions right away. Ignore them until that boss has acquired more rackets. Do NOT run side missions for a boss if you plan to give some of their rackets to another boss when you assign a district - this work is wasted.