A few year's real commercial experience is often enough to at least double someone's productivity, often much more. Why? Familiarity with all the things that aren't pure coding. Understanding requirements/domain knowledge. This is the big one, if you are working in field X and you know the terminology, the business and so on, you can find out what people need and solve their problems with no messing around. Just to pick an example I am familiar with, banking, in pretty much every programming/IT job ad it will say "must have previous banking experience".

Plus there's experience with the toolchain. Experience with the codebase. Knowing how to play "the game" of meetings and reports.