A use case for something like this, and not just for medians, is where you have a querying/investigation UI like Grafana or Datadog or whatever.

You write the query and the UI knows you're querying metric xyz_inflight_requests, it runs a preflight check to get the cardinality of that metric, and gives you a prompt: "xyz_inflight_requests is a high-cardinality metric, this query may take some time - consider using estimated_median instead of median".