Taking inflation into account offsets the fact that dollars in 1986 didn't have the same value as dollars today. That allows you to compare, roughly, how rich different people are in different years.
So you can compare the 100th richest person in the US in 1986 to the 100th richest person in the US today in terms of what they could buy. But the interesting thing, which is how this whole sub-thread started, is asking whether or not the rich are getting much richer even beyond economic growth. So the 100th richest person in the US is richer today than the 100th richest person from 1986, but overall, on average, people in the US are richer today than folks were in 1986, so that direct comparison tells you little.
So how do you account for that? The easiest way is to just compare to GDP, but you need to account for the population difference, so you can just use per capita GDP. Basically you're asking: how much is this amount of wealth relative to the average single person slice of the "economic pie" in a given year.
In 1986 that figure was roughly forty thousand "pie slices" for the 100th richest person, today that figure is ninety thousand "pie slices".