Your read/inbox relays help to protect your received replies from spam & other unwanted things. But, you might want to write to big, public relays so your notes are more prolific. A lot of spam makes it through the big, public relays because spam accounts look a lot like a new user. Nostr overall is currently midway between 2 very different methods of handling the exchange of notes. Eventually, it will matter less where you write, but it will always matter where you read.