The additives have two main purposes. The first is anti-gel. The wax like substances in straight #2 diesel will start to solidify as the temp drops. When this happens the fuel filter plugs up and the engine stops. At this point you will either need to get the machine into a warm place to thaw out or add some of the 911 products. In either case you will probably end up having to change out the fuel filter. In colder areas the fuel suppliers will start preparing their fuel for winter based on the local weather so in ideal conditions you, as the end user, would not need to do anything. However, this does not solve the problem where you bought a 5 gal can of fuel in Sept and are still using it in January.I ask that question about diesels because of the additives in winter. Do you need them? Wat exactly do they do?
The second purpose is for lubricity. The fuel itself is the lubricant for the injector pump. Due to environmental regs the suppliers have been reducing the amount of sulpher in the fuel because sulpher contributes to NOx and particulates in the exhaust. The current sulpher standard is 15 PPM (aka ULSD). The problem with this is that the process to remove the sulpher also reduces the lubricating qualities of the fuel. The supplier is supposed to mitigate this by using an additive package at the tank farm or bulk rack. In the ideal case you as the end user should not have to worry about this either.
Having said all of that, I am not willing to put my faith in the supplier actualy adding their additive package when dealing with my very expensive to repair/replace injector pump and injectors. I generally use enough fuel that using fuel purchased in Sept in January is not a problem
In short, I run additives year round in my tractors (big and little), diesel PU and TDI Jetta year round for the lubrication side of the question.
FWIW - Frank