Well, they are indeed benchmarking the user agent.
The time it takes to perform math functions, which only require CPU horse power, is a metric of CPU performance. The more performant the CPU, the higher the metric (or less the time to complete the cycle).
It is a benchmark. Now the question is: how can the benchmark be used in security/privacy scope?
I have repeatedly looked at amiunique.org and they, being an open source project, describe detailfully what's under the hoods. Truth is that Canvas fingerprinting is a technique that measures the performance of the system to render a canvas to add to the browser's fingerprint. It is possible that the site is using the performance metric to add it to the server-side fingerprinting routine. Who knows without auditing the server. It is interesting to note that Mozilla prompts (or was planning to, anyway it works on Tor browser) for canvas image, so math functions could be used as a workaround to canvas
- Visitor statistics? Very likely
The reputable site may just want to collect info about the CPUs of their visitors. This could be especially true if they want to understand the impact of a front end change or perform browser testing on limited performance machines. Normally web developers have powerful machines with huge amounts of RAM and big screens. They can easily test on multiple browsers, resolution and connection speeds. But what about capping the CPU? Does the site continue to display well? With such metric, they could draw graphs and find test cases
- Targeted profiling? Who knows...
I am getting creative here, don't get me too serious. An advertising company could use such information to understand how powerful is your hardware. Then you can get advertising for the most recent 3D hyper performance game that may run on your machine, or some old classic if they find your CPU is not powerful enough.
Please, don't take the last seriously. I was going to joke on that.