It blocks robocalls. When someone calls you it checks the call against a database of robocallers and blocks known spam calls. It can also screen calls. It works pretty well, but it's not worth $7/week.

Is that functionality not a standard part of ios? The stock Android dialler has had it since 2015...

You can block unknown callers, but it just pushes them to voicemail. Robokiller intercepts the calls and keeps them off your phone completely.