A choreo can be used throughout the process. A couple of reasons. First, a program can - devolve - as the skater forgets intricacies, thus, having the choreo check to see if the existent program is the same as the choreo'd one is useful. Second, as a skater improves (speed, new elements, able to do different transitions) - the program can be altered. Third, based on feedback from comps, particularly under points, the choreographer can adjust the program to reflect the judge's feedback and work to maximize points.
It also lets the coach do what the coach is paid to do - coach - same way that you can use spin specialists, transition/footwork specialists, etc. Once skaters get past the intro level, it's not uncommon; I'm more happy to hear that a coach recognizes that s/he can't do everything, and wants to use someone with expertise.
Plus, choreos tend to be better skilled at cutting/choosing music, which helps avoid inappropriate choices or poor music editing.