It's not a great number, but it has a decent amount of value as it is something we've all been familiar with for most of our lives. When you see a guy with a 2.25 ERA, it generally shows that he has been pitching well. If you see a guy with an ERA above 5, it shows that the pitcher has been knocked around and may not have kept his team in as many games as the fans would have liked.
It's why Price's ERA is spoken of more frequently on here than his FIP. His FIP is only .20 higher than his career average. However, his ERA is 1.2 above his career average.
We can talk about how a pitcher will do in the future, but it's just guesswork. We can definitively talk about the results from previous games.