Layouts that adapt to the majority of web aware devices without significant loss of content. Accessing the internet via mobile devices accounts for more than 20% - 40% of total traffic - a significant section of any market. Not an absolute solution in all instances but, it does allow a broader reach across devices.
Screen Real Estate
There are thousands of different devices capable of rendering websites, mobile phones, tablets but, there is no one size fits all. Images can be reduced in size and loose detail but not necessarily meaning. Text is more difficult, there are limits to scaling and legibility depending on the device.