If you want visitors (customers, clients, prospects, employees, and partners) to continue accessing your content, you will need to ensure that your web site works in mobile environments in addition to desktop platforms.
Most web sites work well on a PC but work poorly on a mobile device.
This is primarily because of the visual layout, but it is also because browsers on mobile devices have fewer capabilities.
Very few forms work at all on mobile devices because many of the tools used to build forms are designed for specific browsers like IE7.