It’s an existence proof. Even at an indie startup, if you mention your idea too early there’s a risk you’ll lose the motivation to work on it due to negative reception. Not every idea that’s poo-poohed is bad.
The trick is knowing which feedback to listen to. If Dropbox listened to the negative feedback from HN on launch, they'd think there wasn't a market and it was a dumb idea. HN was (famously) wrong, partly because HN isn't the target market of the service.
https://news.ycombinator.com/item?id=8863