Email:-

Perhaps stating the obvious - email like post, doesn't demand the recipient is available at the time of sending. However, unlike post - it is (or can be):- Virtually instantaneous; Include text, sound and visual content; Be considerably more secure than a letter or fax - and cost nothing (in itself) to send.

The environmental impact, saving trees, travel & pollution is still underestimated.

The World Wide Web...  enables an effectively limitless method of distributing or broadcasting information, incredibly cheaply. 

The other side of this 'coin', is that almost anything that someone wants or needs to know is readily available.

Beyond that, there is far too much about the Internet and the World Wide Web etc., to even make a start here. So, it seems a good place to stop.