My assumption is that while Nathan is working on this show, he doesn't often need Michelle in the capacity of managerial person, or whatever she's been to him. At the same time, he wants to keep her in his "sphere" so that when The Rookie is gone, he still has her available, and she doesn't have to seek other work during the down time, etc. etc. Thus, it's a bit of a pay raise for him that she's paid by the studio to be an EP. So it's essentially a way to pay him via paying her retainer. I doubt that she does a whole lot in the capacity. She's not a story editor, for certain, and I doubt she's a financial guru. She probably has some day to day projects she manages or helps others to manage.