Software engineering is a tricky topic, as most companies made the error and modeled their structure based on consultancy or law firms, so flat hierarchies, hard jumps in salary based on position, so on.
Especially when you're into contract or project work, you know your wage and what the company takes by renting out your time. So a smart employee will try to "jump ship" to get a raise in salary, especially when internal movement is not possible.
So in this environment, itīs a way to gauge how likely the chance that the prospective employee will leave you later and how fast.