Maybe it does for some people but based on all of the functional specifications I've implemented over the years, they are by far the exception. Maybe designing and documenting first is OK if it's done by people who've already learned how systems work by building and maintaining a lot of them, though.
The documents used in Amazon at that lifecycle stage aren't functional specifications. The point is to justify launching a few project to management. You should be able to clearly explain why it's worth expending resources.
