I'm not sure that's really relevant here, since this is monitoring over time. When you design this system sure you could look at the graphs of data, and you'd s

I'm not sure that's really relevant here, since this is monitoring over time. When you design this system sure you could look at the graphs of data, and you'd still end up with the same impression. The point is that over time you can't collect and analyse all the data always. The important thing here is understand the underlying system and identify what it's modes of failure are. If the likely mode of failure was just to fail to serve some requests at all then the new approach in this article would be entirely useless since the latencies would be the wrong metric completely. In this case they identified one mode of failure is for a small % of requests to deteriorate almost exponentially and so watching the 99th percentile is useful - this is very common in networking. That's great for this system but the overall message is to find the metric that actually reflects the ways your system will fail- and you need to do that in a practical way that may not necessarily involve recording and storing all the data.