1) From a user experience / marketing standpoint - (target demo's have a better grasp of their own likes and "dislikes" when engaging an application.) - if one of the targeted demo.
There are "discoveries" that usually happen through conversation. A mature programmer can provide an entrepreneur perspective or another idea to improve the product due to the architecture of other programs (built in the past).
2) Depending on the Programmer; the older way of training code revolved around meticulous detail) - also very helpful sometimes in avoiding bugs.