I understand that I can use this constraint in MS Project, but really, what is a good use case for this? When does this constraint make sense?
A use case might be procurement of a service/software that is a pay-as-you-go service that won't be used until a certain release point. You want to wait as long as possible before procurement to ensure you're not wasting money up front.
There are a lot of other use cases.
This constraint makes sense when you have a task with any of the following characteristics:
- It can take place at any point in time, up to a point, but there is a future event/date/task by which it must be done
- There are higher priority tasks which you would prefer take place before said task, but which are not literal predecessors of the task
- There is an external reason why it makes sense to leave the task as late as possible (such as that outlined by mwan)
As an example, I commonly use it for all pre-deployment preparation tasks. The actual deployment date/weekend/milestone/task happens at a time driven by the bulk of the project plan, but the tasks to prepare the environment, complete the service transition documentation etc. can happen at any time prior to the deployment; but clearly you want them just before deployment. There is no point preparing the service transition documentation at the beginning of the project, you want it right at the end just before actual deployment.
So the backloaded tasks becomes a pre-requisite for the deployment milestone/task using As Late As Possible, which means they butt up against the deployment tasks and don't get scheduled earlier in the project.
A simpler reason is if you are dealing with something that has a short expiration window, such as fresh shrimp or ice without freezer space. You would want the delivery to come "Just-in-time" or as late as possible.
Use it in power plants to hold off building scaffolding to repair a valve in the overhead. If you tie the scaffolding activity to the valve repair with ALAP, it will always lead the valve repair no matter where you move it. We do not logic tie a predecessor to the ALAP activity.