Stay interviews are an opportunity to get feedback for employees about what they need to stay. Employers and managers should think of stay interviews as engagement discussions that supplements everyday interactions, survey feedback and individual motivation.
This is an opportunity to show top performers that they're valued and ensure that you're investing in them to avoid an exit interview.
DISCLAIMER: Be prepared to hear hard things and make sure you're prepared to act on feedback. This is an opportunity to build trust so don't squander it with inaction.