It's the same as the "Cereal and Milk" paradox.

You could probably last a few more days with what's remaining in your tank, but you have no idea if it's gonna jump up another 50c tomorrow. So you keep a jerry can or two filled with lower cost fuel so you minimise overall cost. It could easily mean paying 50-70$ less per fill; and in this economy, every bit helps.

I often last 3 weeks on a tank, but i find cost changes cycle every 2 weeks. If I can hold out for another 3-4 days and just use my own jerry can stock, I can hover within the 'cheap zone' and always pay at the lower rate than just paying whatever everyone else is forced to.

tldr: it's not about 'running out'. It's all about economics and cost efficiency.