Many possibilities really. Lightning gossip, coinjoin protocols, website access, nostr relays, basically anything where you want to restrict access with scarcity (to prevent resource exhaustion), but want a very bar of anonymity for the user and also convenience (payments arguably may not meet this criteria).

Earlier description of the same broad concept here: https://reyify.com/blog/riddle

The difference here cf the original proposal is that this has very fast (sublinear) verification for the server/verifier, making it practical for v. large anon sets.