This part is a concession to fate, and a reiteration of the dichotomy of control.
Remember that thou art an actor in a play, of such a kind as the author may choose; if short, of a short one; if long, of a long one: if he wishes you to act the part of a poor man, see that you act the part naturally; if the part of a lame man, of a magistrate, of a private person, so the same. For this is your duty, to act well the part that is given to you; but to select the part, belongs to another.
A belief in fate or a higher power isn't required for Stoicism, but it can be an interesting way to frame things to illustrate the limits of our control. And whether you believe in predetermination or not, the take-away message is the same: you don't get to choose the part your play, but you can choose how well you play it.