multi content-type clients that treat every event kind as a first class citizen like Ditto and @Nostria are good candidates. the use case is user-created custom feeds or counts, filterable by you and your contacts' pubkeys (you can do not so common stuff like querying what your contacts are zapping, reacting to, etc.). spells solve the problem of feeds being hardcoded in clients and a closed system. by making REQ queries reusable we make clients open and expose ways to see the network that were not possible before unless client devs hard coded them. hope i answered all your questions, if not let me know. i read this by pure chance since it wasn't delivered to my inbox relays btw.