Optimising web content for the thousands of internet capable mobile devices has always been a black art. A number of open source libraries exist with the aim of maintaining a list of all the mobile handsets and their browsers, including algorithms for trying identify the capabilities of those phones.
From entry level services using GSM modems or bluetooth enabled phones, through to web services taking advantage of cheaper international SMS rates or a direct SMPP connection into a mobile operator to manage short codes and billing directly.
SMS is often seen as a way to interact with a user when they aren't actively using your website or application. Alerts, warnings, special offers all re-engage users and drive traffic back to your website or application.
These "widgets" often even have access to other features within the phone itself, like contacts, position information and calendar entries.