Searching popcount in Python files on GitHub yields
a considerable number of examples:

https://github.com/search?utf8=%E2%9C%93&q=popcount+extension%3Apy&type=Code

Perhaps intresting:

* In CPython itself: See count_set_bits in
  Modules/mathmodule.c

* Domain specific applications: Bitboards in Chess,
  fairly shuffling cards in Poker, comparing molecules

* Size of bitsets (see bitarray and bitsets I listed above).
  Probably for this reason also as a first class citizen
  in Redis: https://redis.io/commands/bitcount.

Probably most important:

* As the Hamming Distance:
  https://en.wikipedia.org/wiki/Hamming_distance#History_and_applications

---

Btw. not a concrete application. I just stumbled upon this.
popcnt was considered important enough to be included in the
rather limited WebAssembly instruction set:
https://github.com/WebAssembly/spec/raw/master/papers/pldi2017.pdf