(This is one of a series of Software Engineering Maxims Which May or May Not Be True, developed over the last few years of working at Google. Your mileage may vary. Use only as directed. Past performance is not a predictor of future results. Etc.)
For any given task, there is often some person somewhere who has done it before and knows exactly what to do and how to do it. For things which are likely to be done once in a project and never repeated, relying on that person (either to do it or to provide exactly what to do step by step) can significantly speed things up.
For things which are likely to be repeated, or refined, or iterated upon, it can be better to not rely on that one expert. Learning by doing results in a much deeper understanding than just following directions. For an area which is core to the product or will be extended upon repeatedly, the deeper understanding is valuable, and is worth acquiring even if it takes longer.