What is ECOxxxx

ECOPlus We fortify every gallon of heating oil we deliver with
ECOPlus, an additive that dissolves impurities that occur naturally
inside your tank. If allowed to accumulate, these impurities cause your
system to run poorly. With ECOPlus, not only will your system use less
heating oil and produce fewer emissions, used consistently, ECOPlus
reduces system wear and tear, and helps lengthen system life. We provide
ECOPlus at no additional cost to you! Get pricing and delivery info.

Does this make sense?

If the impurities accumulated in the tank, what harm could they do?
They're in the tank.

Instead they dissolve them so that they go into the furnace. Is that
really a good idea? I thought they were bad.

Or are there no impurities?