it's a great form of communication, and a great platform for communication. That's pretty much it, but that is a BIG deal. Think about how the telephone changed history, being able to talk to just a single person. Now, multiple that by the number of people you can talk to on the internet (I don't know of this is mathematically correct, but it should provide some insight on how useful).
Someone in communications may be able to give deeper into that, but that's not my field.