The sprint forces a "this is what we're doing in these 2 weeks" with a deliverable at the end. If, at the end of the first sprint, you only got half the things done because of challenges, ok that's what it is. Next sprint you pick up only half as much work and you get that done.
Now, the PM can look at all the work that needs to be done and the rate of work that needs to be done and the amount of work and realizes that instead of the initial "yea, we can do this MVP in 3 months" that you're closer to having it out in 8 months if everything remains the same rate.
That is something actionable for project management to say "nope, can't have the MVP in 3 months with this scope" after only 4 weeks and the project can get canceled earlier.
Sprints try to make management realize overly optimistic estimates and timelines and either have things get adjusted (staff, scope, or time) or have it canceled sooner. Having a sprint based project canceled after 4 weeks rather than embarrassed and dragging out at 12 weeks is good for the organization to not spend resources on things that won't get done in the required timeframe.
The goal of the short framed boundaries is to force this decision sooner.
And the third paragraph kind of lays bare the problem with sprints: The PM can only look at all the work that needs to be done if, well, there was a discovery phase to list all the work that needs to be done. That discovery looks suspiciously like planning.
That's why agile focuses on continuous delivery of value. It's excellent for projects where it's hard to estimate the full scope, but there are many intermediate targets that are valuable to the customer. You go along until enough value is delivered, and then move on. Not all projects work like that. Some things only generate any value at the end of a long slog. Some things can be delivered incrementally but require a predictable end date. (Hello, Black Friday!)
And so, you get to adopt methodology to the problem at hand. Sometimes, you really need to spend the planning up front. Sometimes it's enough to sketch it out on a napkin. Sometimes you can do things sprint-by-sprint. Sometimes, you have larger checkpoints and sprints in between. The idea that any single methodology works for every problem is an idea favored by the consulting industry, but not a reality. And it predates agile, by a lot - I've been a process consulting victim way back in the early 80s. (And the mass of miracle methodologies directly led to "No Silver Bullet")
When done well, any iterative method is meant to provide early stakeholder feedback at each stage, provide some measure of progress, and have earlier warning signs that the wrong thing is being built.
> What the hell is the point of the short time framed boundaries?
It's easier to estimate a simpler and smaller thing than a larger and more complex thing. If you cannot achieve the small thing, you absolutely won't be able to do the larger one.
