The way I see them: 50+ stars means people use the code. If there's also a decent number of issues opened and closed with comments from the repo maintainers, I'll be much more likely to try things out. Less than 50 means I'll read the code to see if any ideas are worth adopting, but I'm not going to depend on it.

If a project has 500 stars, I can't tell how many are from the project's value versus the maintainers' popularity. GitHub is also a social website, so there's no avoiding it.

Its a good indicator how well the source is advertised. But at soon as its used as a metric it becomes useless as its easily gamed.

Not the strongest business metric but it shows developers’ attention and a signal that people somehow care or want what is built.

The strongest metrics would be revenue, usage in production for large companies, big contributions that are themselves use for real life use cases.

Vanity

Pretty relevant for an open source project.

A lot of GitHub stars on some popular projects could easily get you credibility when applying for jobs, and even be a deciding factor over other candidates.