For one, not having to manually check is huge. If someone posts once every two months, I’m not going to manually open their website every day (times twenty for every one) or even every week and then have to think “have I read this?”. That would be a colossal waste of time, attention, and mental energy.

For another, there’s a plethora of cases where RSS filters. I subscribe to YouTube channels exclusively through RSS, I never open the website (or even have an account) otherwise. Same for Reddit, they have specific feeds for each subreddit, and for each they can even list just the top daily, weekly, monthly, or yearly posts. I can easily get just the most interesting posts of something like r/toolgifs without ever having to deal with the faff or getting distracted by other junk.

Content comes to me, and on my terms.

It’s content only.

It’s also a drastically simpler standard than HTML making creating RSS clients far easier than an HTML client.

Finally it is designed to push new content in a way HTML isn’t. I can tell my RSS client to only download all the titles for all the different posts and only download the content on actually clicking through. Thats something HTML cannot do. Further, my client can easily retrieve changes since it last checked which HTML doesn’t allow.

Lots of differences. Overloading HTML which even at its simplest is about fixed text and overloading that with handling temporal lists of atomic data would be a bad idea.