Mobile means a small device which therefore has a small screen. On these small screens, websites designed for larger devices can look unpleasing to the eye and are completely inappropriate for the device and by extension, the user.
Some websites have brilliant mobile versions of their sites, but some don’t. If you go to these websites that aren’t designed for mobile on your phone then you will probably find it difficult to use the site. This is a big problem as it deters users from using the site and therefore potential revenue will be lost!