Much like Socrates (who in the Stoic tradition was the quintessential ethical standard), Epictetus regarded the pursuit of philosophical contemplation and reason as something intrinsically practical, as a way of actually understanding life and living it in order to attain its highest treasure: Happiness.
It would be overkill for this particular post to review the system of ethics held by the Stoics. Suffice it to say, however, that Epictetus constantly warns that there are things that are in staunch (and sometimes brutal) control of external forces, and that, if we want to achieve anything resembling a happy and content life, we ought to allow Fate to reside over those things.
So when thunderstorms come, or illnesses arise, or death or the conflagration of the world unfolds, we must not shrink beneath them in fear, but rather know that though these things be, there are things that we in fact do have control over- namely, the way we view events, how we respond to them both emotionally and intellectually, what we ought to fear and what not. In short, the interiors of our mental states, which neither thunderstorm, nor illness, nor even death have the power to assail. Hence the poem.
As a side note, while it is true that the ancients believed in the Moirai (i.e. the Fates), it doesn't follow that the Stoics did. However, they most certainly believed that, while we have free will, the universe and all in it are still governed by Fate.