Scrum is iterative and incremental. While you plan a sprint, you loosely plan an increment and using the estimates and velocity, project where you will land.
It can be useful to know you're not going to make the 6 month project 3 months in. So you can readjust expectations or abandon the project entirely.
People will listen if you have evidence and the team on your side. Arguably this all depends on your project, culture and product.
> While you plan a sprint, you loosely plan an increment and using the estimates and velocity, project where you will land.
That sentence contains four verbs: plan, plan, estimate, and project. If your sprint were a single week, planning two things, estimating all the things, and "projecting" the sum of all the foregoing adds up to... Quite a lot.
You'll spend a large portion of your time doing these... fundamentally non-agile things.
Scrum is basically Waterfall, packaged as a lot of mini-waterfalls and sold as "non-Waterfall".
You can't run a marathon as an endless bunch of sprints; a marathon is fundamentally different from a sprint.
It can be useful to know you're not going to make the 6 month project 3 months in. So you can readjust expectations or abandon the project entirely.
People will listen if you have evidence and the team on your side. Arguably this all depends on your project, culture and product.