It’s also how browsers detect a website supports HTTP3. Browsers will request it just to check if they should connect to an https:// URL via HTTP3 (though they generally don’t block on it - they fallback to HTTP1/2 if it takes too long).
It's one way, but a H1/H2 connection can also be promoted to H3 via the alt-svc header. The DNS method is slightly better though since it potentially allows a client to utilize H3 immediately from the first request.