Bitbucket build status is send every time a new build has been started and every time a build ends (stopped or completed does not matter).
The build status is associated to a commit and helps user to understand if the commit is good or not (regardless the branch is which is). It is possible in Bitbucket Clous and Data Center enable some feature based on build result that prevents merge for pull requests or other things.

The build status contains a set of informations about the Jenkins build. Some of these informations are not provided by design by the bitbucket-branch-source plugin. As user I would be able to enrich the build status with extra informations maybe provided by other plugins (like unit test, coverage etc etc) to enhance the user experience.