Selenium solves a different problem. With selenium, you record user actions, and verify properties about the state of the site.

The record/playback feature is designed to make a copy of the content you look at during a session, allowing you to retrieve it offline at a later time by revisiting the page (or making equivalent HTTP calls).

Interestingly, if you use both together, you can design a offline-mode reader for hacker news for use on the train or other disconnected environments. Or, combine selenium and playback mode to create kick ass demos that are less likely to fail on weak wifi networks. :)