It's less to get and act on a concrete answer to the question, but rather that asking the question unambiguously signals openness and desire to improve. As long as the recipient of the question believes it's being asked earnestly, it doesn't ever need to be directly answered.

Part of the benefit is that setting aside time to talk about what you can do better forces you to think about what you can do better. Sure, some people will think about that without needing prompting, but having a set time to do so is beneficial for everyone else.

Additionally, it gets the employee and his or her supervisor on the same page -- "here's what we agreed I would work on, here's how it's going."