I believe that Glassdoor exists almost purely because salary information is not public. While company reputation and reviews are of interest, the salary information is likely much more a driver of traffic.

And based on my attempted use of the site, Glassdoor is unreliable and (intentionally) misleading.

Even at the most basic level, their UI flatly lies about what data they possess. If you search a company, in the salary column it will show a number which is designed to suggest a number of salary data elements available. Honestly I cannot say what that number actually means as it seems to have no value at all. Specifically, it will say 5 or 8 or some other small but non-zero number... and yet there are zero visible salary records for the company (even if you are a contributor of information and thus deemed to be allowed to view other information on the site).

I expect Glassdoor is like the many dating sites on the internet - full of fluff and false/artificial information so the site appears to be valuable and worth visits.

If we ever reach a point where company position salaries are public on a broad scale, Glassdoor will fade into obscurity.