Difference between the 4 Built in RSS Feeds?

Feeds are not stored, WordPress generate them on request. That means that even if WordPress can generate 4 type of feed, that does not affect in any way performance, but gives the possibility to users to choose a format they like according to reader they are using: is not WordPress fault if there are different types of feed standard.

The WordPress preferred way is RSS 2.0, but if an user has different needs, WordPress allow to choose rdf, rss 0.92 or atom.

Of course you, as a site owner, can decide to support only one version, it’s as simple as:

add_action( 'pre_get_posts', function () {
  if ( is_feed() && ! is_feed( 'rss2' ) ) { // only support rss2
    header("HTTP/1.0 403 Forbidden");
    exit();
  }
});