Mobile phones have developed so that they have enough bandwidth and screen size and resolution to access the internet; but many web sites are still designed for desktop or lap top computers with screen sizes upwards from about 38cm (mobile phones have a screen size of about 9cm).
If you have accessed the web from a mobile phone browser you will find for many sites you can only see part of the website, or that it displays as simply a very small version of the desktop site. Typically mobile phone browsers are not able to see flash (animations), java (commonly used for links) and interactive components. In addition web sites not built for mobile phones like iphones and Androids usually have more text and bigger images, making them unwieldy on a small screen.
With more mobile phones than people in Australia, a growing number of which are able to access the internet, it is good business to have a mobile web site for your business.