Adding fuel system cleaner to your gas tank

Why do directions for fuel system cleaners say to add it to an almost empty tank then fill the tank with fuel? What’s the difference between doing that and adding it to a half empty tank and then filling up the tank?

Possibly to mix the cleaner up with the gas. Try this, put a spoon of sugar in a glass then add hot coffee (you almost don’t have to stir it). Then fill a glass with hot coffee and pour a spoon of sugar in it (it just sinks to the bottom and you need to stir it)

Slightly less effective mixing.