When you keep stakeholders updated early, you force them to make hard decisions about their product so you know what to prioritize.

It makes everyone focus on what is most important.

Are those features really necessary for the release? Can we change the requirements to accomplish the goal easier? Is it important enough that we should delay it?

Without a deadline, engineers waste time working on stuff that is fun and interesting to them and not the hard stuff that is needed to ship a product across the finish line.

I do that along with every other engineer I know. It is human nature.