One thing a culture of reproducible builds (and thus stable hashes) however does provide is a lack of excuse as to why the build isn't reproducible. Almost nobody will build from source - but when there's bugs to be squashed and weird behavior, some people, sometimes, will. If hashes are the norm, then it's a little harder for attackers to pull of this kind of thing not because you trust their hash rather than their blob, but rather because they need to publish both at once - thus broadening the discovery window for shenanigans.

To put it another way: if you don't have reproducible builds and you're trying to replicate an upstream artifact then it's very hard to tell what the cause is. It might just be some ephemeral state, a race, or some machine aspect that caused a few fairly meaningless differences. But as soon as you have a reproducible build, then failure to reproduce instantly marks upstream as being suspect.

It's also useful when you're trying to do things like tweak a build - you can ensure you're importing upstream correctly by checking the hash with what you're making, and then even if your downstream version isn't a binary dependent on upstream (e.g. optimized differently, or with some extra plugins somewhere), you can be sure that changes between what you're building and upstream are intentional and not spurious.

It's clearly not a silver bullet, sure. But it's not entirely useless either, and it probably could help as part of a larger ecosystem shift; especially if conventional tooling created and published these by default such that bad actors trying to hide build processes actually stick out more glaringly.