Because as a lead, they may know a lot more about something, or have experience with it, or have thought about the specific problem space a lot more. Sometimes

Because as a lead, they may know a lot more about something, or have experience with it, or have thought about the specific problem space a lot more. Sometimes it's that classic, "I can do it in 4 hours, but you will take 2 days. You do it, and learn something, and next time we'll be equals" kind of thing.