If your car's fuel is going to be sitting in the tank unused for a long time, adding a fuel stabilizer is justified. When your engine is running unevenly or uses more fuel, an injector cleaner may justify its cost if it manages to solve the problem. However, not all running problems can be solved with additives; for example, high diesel consumption is often caused by clogged air filters, which you will need to replace rather than just adding something to your fuel tank.