As Brian Moore points out, an extra digit is an easy visual way to filter out higher prices:
Furthermore, we save on mental transaction costs by processing the more signicant digits first and often ignoring the rest. That fits our habit of scanning numbers from left to right.
If I see the following prices for gas:
My brain immediately dismisses the 3rd price, then goes back and looks at the others to find the lowest A seller doesn't want to be eliminated like that.