I bought one on Amazon when we first got the rivian before a trip to Vermont. But the two superchargers we stopped at had the adapters built in. So I returned the Amazon adapter when we got home... Rivian eventually sent us one for free, and I've used it once in the last year, can't remember what trip I used it on. For longer trips, it's certainly nice to have, Tesla's network is more reliable and robust than anything else. For everyday use you shouldn't ever need it, just plug it in when you get home.
Also, the adapters are different for level 2 vs level 3 chargers. I had to get a level 2 adapter (nacs to j1772) to use my Tesla wall charger with the rivian. And the level 3 adapter (nacs to CCS) is for the superchargers.