Newsletters =/= RSS feeds. Newsletters provide a direct way to talk to your audience about subscriptions, fundraising, merchandise, etc. in a way more effective way than RSS feeds provide. Additionally, newsletters require me to give them my email address, which they can use for promos and all sorts of purposes (or sell off). That’s not even getting into all the granular data they get via trackers and suck in newsletters.

An RSS feed is me grabbing a periodical off a stand. A newsletter is giving them my address and mailbox.