AppCache enables something no other platform does: use the app immediately, download it for later, offline use in the background as soon as it's loaded.
As such, not only does it provide a great solution for building offline applications, it also positions the Mobile Web Platform as the most compelling platform to deliver short-lived and highly-local applications, which no other platform is in a position to do effectively at the moment.
Unfortunately, AppCache suffers from a number of small flaws which make developing for it very difficult. Some of these were discussed and listed during The Future of Offline Web Applications Workshop that was held in Redwood in late 2011. In turn this triggered a number of changes in the specs and in some implementations (notably Microsoft's).
Not all issues have been fixed, however. Also there are some quality of implementation issues which would be worth documenting as they often trigger feature request, e.g. being able to defer AppCache automatic upload on network heavy apps.
AppCache is part of HTML5.