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 14cm).
If you have accessed the web from a mobile phone browser you will find that on some sites you only see part of the page, 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, it is good business to have a mobile web site for your business.