Mmmm… read the book on Extreme Programming.

The idea is that you let the code drive the system and do not optimize prematurely. Sometimes developers design parts that are not needed, and often not in the first phase.

It is a way to evolve a system unbiased. Of course there is a trade-off. If the refactoring/change is very expensive, like a database schema change in production, it is good to spent a lot of upfront design. Takes experience to see where you can relax and where you need to be careful.