They do. You get signals on how well people can work independently on a non-trivial task. How well they can understand their is being communicated to them, whether they clarify what is important, etc. How well they can get themselves unstuck vs needing to ask about everything. How motivated they actually are to work, etc.

They're not perfect, and you really do need to make sure the task is targeted at what you actually want to see from the person when they join, but they definitely give more signal than 6 hours of leetcode.