It's just a term invented so people can make themselves look silly and quote them during 1000% bull markets. They have "some" value at the bottom of bear markets, but to make it even more laughable people quote trailing PE under some delusion the market trades on what has already occurred.
It's a little more respectable to look at projected earnings in the next 1-5 years and then extrapolate what you are willing to pay for those earnings, but that would still be of less value than looking at cash flow for the same period.