The key is in the name, mobility. Being able to look at a map on the web on your home computer is nice, being able to look at map anyplace your phone works is the so much better.

Internet in your pocket.

Go watch the original iPhone launch keynote with Steve Jobs. Before the iPhone "mobile internet" was a completely separate version of the 'net - if a site even bothered to create a WAP version of themselves to render on phones. Before the iPhone the only thing consistently useful on a mobile "smart phone" was email. Having a real web browser that rendered real web pages was a watershed moment!

I guess the introduction to 3rd party apps aka App Store.