Ask HN: What are 'deploy previews' (ephemeral environments) good for?

Does anyone use deploy previews (AKA 'preview environments' or 'ephemeral environments') on a regular basis to see/share github pull requests?

What platform do you use for this?

What are some use cases/workflows in which this capability helps you the most?

What's missing from these solutions that you wish they offered you?

Thanks!