A historical PE may be (and is most likely to be useful if it is) based on adjusted EPS.
Although future performance is what matters for valuation, and investors would normally look at prospective PE or other forward PE ratios, the historical PE does have the advantage of being usually known with certainty. It can therefore make a useful starting point for investors; something they can look at before considering growth.
Some, more conservative, investors prefer to use a long term PE. This is a historical PE calculated using the average EPS over several years.