Motivation: I think employee turnover is a great measure for employee satisfaction, especially if you believe in the dead sea effect where a company's talent leaves more readily, causing accumulation of less capable members.
Objective: Knowing the employee turnover would allow applicants/ prospective employees to make a more informed decision. I guess the easiest way is to ask employees/ management, but asking a metric like this is unlikely to be consistent across companies, and very easy to lie about. I'm happy to accept strong alternatives to employee turnover as a measure of employee satisfaction, though Glassdoor reviews are particularly unhelpful for smaller companies which have been around for many years. (Only a small group of people write on Glassdoor).