With the source code integration you can view the commits that has been done for an issue. In the Edit issue page you can see the commits that has been done for an issue.
Simple forms is a way to have "input forms" outside of VisionProject and having issues created in VisionProject.
It can for example be used from within your own website to get feedback, report bugs, ask questions, etc.
A Web service API exists so you can have external systems that update, read or delete data.