Versioned builds are one of the more confusing features of Codename One mostly because the word "version" is so overloaded with meaning. The feature itself is remarkably simple as explained in this video tutorial.
Versioned builds allow you to build against a fixed point release version of Codename One, that means that if we break something in the build servers you can still work against the last major version that we released.
This is also useful in cases where you want to see if something failed recently and also for getting consistent build results so you can update Codename One versions at your own pace. There is a limit to how far back we can support. The mobile world moves very fast so keeping support for older versions of Codename One is just impractical in the long term. However, this allows you to slow down the fast pace of progress a little bit.