The idea is that if data is being added from a completed form, you can create a rule that allows the user to handle that item differently then you would otherwise (if a task was added manually from within the project).
For instance, if you have a Rule setup to have new tasks moved to a specific section, that might not be expected behavior when items are being added manually. You might not want to temporarily disable the Rule because at the same time users may be submitting items through the form. If Asana is able to distinguish how an item is being added, I could see this added functionality being really beneficial while also enhancing the workflow.