[srg0]

Percentiles are order statistics, they are robust and not sensitive to outliers. This is why sometimes they are very useful. And this is why they do not capture how big the remaining 0.01% of the data are.

Let's take a median, which is also an order statistics. And a sequence of latency measurements: 0.005 s, 0.010 s, 3600 s. Median latency is 0.010 s, and this number does not tell how bad latency can actually be. Mean latency is 1200.05 s, which is more indicative how bad the worst case is.

In other words, percentiles show how often a problem happens (does not happen). Mean values show the impact of the problem.