They are extensively used in Japanese computing. E.g., when you start typing numbers without first switching off the input method editor (which allows you to se

They are extensively used in Japanese computing. E.g., when you start typing numbers without first switching off the input method editor (which allows you to select Japanese/Chinese characters), by default (*possibly depends on platform), you will get full-width numbers as the first conversion candidate. On Japanese sites, HTML forms also often expect either full-width or half-width letters and numbers (in, e.g., addresses). For this reason, it might be useful to generally be aware of them even if you don't use CJK.