1) save significant amount of time ahead
2) give your customer time to identify what they really want
3) help you iterate quickly on newer versions. If something doesn't work, it's easy to throw stuff away
It's a process problem and you can't solve it bottom-up.
You either get used to it and work to the specs, knowing they will change a lot, or you try to get closer to whoever makes the decisions. More often than not, that leads you to another company working on a different problem ...
Old army observation. No Battle Plan Survives First Contact With The Enemy. Thus the sooner you get that over with the better.
Mike Tyson said it better: https://i.imgur.com/Gy9TWiq.jpg
Just wanted to add a source: https://en.wikipedia.org/wiki/Helmuth_von_Moltke_the_Elder