They need them because they all have their dependencies that they need to manage separate from this project. A client may need to plan training, transitions, budgets, deprecation of old systems, etc.

Others above me have mentioned the reasons why they need them. But I'll reiterate my emphasis on rough/relative timelines. Exact dates are impossible and the farther away a milestone is, the hazier the date is. As an example, a milestone a month away could be +/- a week, but a milestone a year away could be +/- three months.

Yes. They need to not sell the next version too soon, but start selling soon enough to develop hype. They need to decide if they should invest in updating the previous solution one more time while waiting... They need to decide if the whole is worth the cost or should they develop a different project instead.