Integration tends to take more time than expected, so ensuring the system works end-to-end in the beginning of the project avoids surprises and schedule slips late in the project. In other words, it's one of the Strategies for attacking project risk.
James Hamilton Get the skeleton version of a full system up early in the development cycle. It gets developers thinking at the user level from the very beginning.
‣ Tracer Bullets
Build end-to-end, not top-down or bottom-up.