[handrous]

IMO the best use of "agile"-style planning is to replace the estimation process with development. After 6-12 weeks you'll have some amount of working (if minimal) software, and a decent (by software planning standards) idea of how long at least the first major set of features will take. If you like what you see so far, and the estimate doesn't seem like it'll wreck your budget or timeline, you keep going. If not, you reconsider things or stop.

...Or you can spend that 6-12 weeks just estimating, involving more time by more people, have only a somewhat-better idea of how long the first set of features will take, and no working software to show for it.

In practice, however, I find few businesses willing to either have a 1.5-3 month estimation window, or to start development with none but a wildly vague estimate, amounting to a guess, waiting 1.5-3 months to find out what a somewhat-accurate estimate may look like.

[regularfry]

The unfortunate truth is that as soon as someone in the process wants an estimate, rather than wanting to see delivered value, you've already got a situation in which "agile"-style planning is on the back foot. Aside from any business value there may be in the estimate itself, insisting on being given one is a political move designed to put software delivery organisations on the defensive: the framing is that IT is a cost centre, not a value source. That's so common that it's easy to forget that it's not the only choice.