From an interview with one of the developers on the twitter.com team:
I think the real power of Twitter is its ability to channel over different mediums at the userâ€™s whim. IM, SMS, email, and the web are just transports as far as Twitter is concerned. Generally, you have to go out and get information via whatever medium that information is on. With Twitter, information can come to you via whatever medium you prefer. Or, if you want some space, you can easily turn off the information tap with a simple â€œoffâ€ command. Thatâ€™s powerful.
I linked to a blog post by Tim O’Reilly a couple days ago that summarized this feature by calling it a ‘communications multiplexer’… There are other companies that do interesting things in this space in different ways: rasasa.net, zaptxt.com, feedcrier.com, etc… It’s also one of the ways I’d like to evolve the instantFeeds plugin I wrote: be able to send an email, IM or an SMS or maybe even message into a web page: get only the information you want, delivered using the medium of your choice.