Back when the internet first began way back in yesteryear, there were many protocols (i.e. ways of transferring data). There was telnet for actually logging into command shells on remote servers; there was FTP for transferring files to and from remote servers; there was Gopher, which provided a very user-friendly system of menus to navigate to get to various information; and there was HTTP for requesting these newfangled documents with hyperlinks in them. Because of the interconnectedness of these hypertext documents, Tim Berners-Lee and Robert Cailliau coined the phrase World Wide Web. There was a previous long-standing practice of naming servers by the internet service they provided, so FTP servers had a "ftp." prefix, Gopher servers had a "gopher." prefix, etc. So naturally they started naming these "web" servers with a "www." prefix. Very quickly, this web of hypertext documents took over as the dominant protocol for most users to access information over the internet. I would guess that 99.9% of people that use the internet these days don't even know that other protocols have – and still do – exist. I still have to use FTP occasionally at work to download product catalogs from suppliers, and I use FTP to update my blogging software, and your email client, for those of you who haven't completely jumped to web-based email clients, is still using POP or IMAP and SMTP, but no one uses Gopher to search for airline tickets or Costa del Sol adventures. It's almost all HTTP these days.
The World Wide Web is the only thing I know of whose shortened form takes three times longer to say than what it's short for.
— Douglas Adams, The Independent on Sunday, 1999
In Spanish, it's even worse, since each W requires four syllables with uve doble. Stephen Fry has suggested we say "wuh wuh wuh", but that sounds like a dog barking to me.
Given the ubiquity of hypertext "web" documents, for me, it comes down to this:
The fact that you have to put the http:// before all of your web addresses anyway already tells your browser and the server and everybody that you're looking for a hypertext document, so the "www." is entirely superfluous!
But old habits die hard, even if they are a waste of resources. Just think of all the bandwidth we are dedicating on a daily basis to sending those four bytes to represent the "www." across the wires!
Even Tim Berners-Lee, the creator of the whole darn thing, said back in 1999 that the WWW prefix was outdated:
Nowadays, however, the web server may be far and away the biggest service foo company has, and it might make sense to give it pride of place. Remember you can only do this with one service. You could use http://foo.com/ which is after all easier to type, even though people expect to have to type the "www".
This issue seems even more important now that we're in the age of Social Networking in which web links are passed around in dizzying quantities, giving rise to an entire industry of URL shortener services.
I hope that more and more companies will figure this out and buck the trend. It's easy enough to forward a URL with "www." to one without it, so your customers will still find you. In the mean time, you can find me at erikras.com, not www.erikras.com.