Whether it’s biodiesel, regular diesel, ethanol-blended fuel, or even home heating oil, fuels made from petroleum do not have inherently infinite storage lives. They are subject to the vagaries of environmental effects like oxygen, water, metallic catalysts, and hungry microbes that would like nothing better than to bed down in their storage tanks and call them home.
Same thing for the storage tanks that these fuels call home. Sludge buildup (and the associated filter plugging) in the bottom of storage tanks is not uncommon. And when microbes and water are in play, you can have the real and expensive problem of tank corrosion.
Consider, too, that these problems can be out-of-sight, out-of-mind. Which means anyone with fuel storage tanks can be on the verge of critical problems and not even know it. Microbe testing and monitoring can be as important as having the right treatment solutions when these problems appear.
How much of a problem these things are (and how important microbe testing and monitoring are) depends on who you are and what you’re doing with the fuel.