The main difference here is one of conscious choice of provider — in the other solutions noted herein, a user consciously communicates with a given server or provider and receives events as pre-determined. Under the Pub-Sub scheme, the user only specifies which class they wish to be part of and what events they are interested in receiving. From there, they receive these events when one is pushed out.
A way this is often framed in internet discussions is in the frame of a radio channel. Record companies, or publishers, issue audio to the station, which then broadcasts this audio to listeners, or subscribers. Pub-sub is the middleman radio station here — listeners don’t know who gave the station the music, nor do the companies know who the listeners are. It is this segmentation that is baked into the pattern.