I was going to mention it too! The really cool thing about reservoir sampling is that it can be done "online" (ie process input incrementally) which makes it super useful when you want to compute statistical properties of something in the field without blowing up your cpu and memory.

For example, let's say I have a server serving queries. I want to measure min/max/avg/stdev/99p you name it. You can do it cheaply with reservoir sampling, without having to save all data points.