Recall: a Stoic achieves his own good because it is entirely up to him. Moral virtue and excellence of character are the sole good. The rest are indifferents to him. While some may be preferred indifferents, they are nonetheless not needed to achieve the good.
Observe: what other people desire and pray for. I'm not familiar with all Christian religions, but having lived Mormonism for over 30 years, I was taught and I believed in what I have come to know as "the prosperity gospel." In sum, it is the belief that God will help you prosper (i.e. crops, wealth, etc.) if you obey the commandments. One of the books of scripture for Mormons is the Book of Mormon and in that book, this teaching is repeated over and over again. The first instance comes from early in the book: "Inasmuch as thy seed shall keep my commandments, they shall prosper in the land of promise" (1 Nephi 4:14). For all the other references, one may perform a search on their website using "prosper in the land" and most search results will reinforce the teaching of obedience to God's commandments leads to prosperity (i.e. wealth).
Observe: if a Stoic would pray, he would pray for greater courage, wisdom, justice and temperance. He would pray to see the world as is really is and that his will is the same as Nature. A Stoic would probably not pray to God for wealth and prosperity - he would not pray to God to grant him indifferents, preferred or not. In fact, we can read Cleanthes' Hymn to Zeus and in five particular stanzas, glean correct reason.
..., Thou canst
Make the rough smooth, bring wondrous order forth
From chaos; in Thy sight unloveliness
Seems beautiful; for so Thou hast fitted things
Together, good and evil, that there reigns
One everlasting Reason in them all.
The wicked heed not this, but suffer it
To slip, to their undoing; these are they
Who, yearning ever to secure the good,
Mark not nor hear the law of God, by wise
Obedience unto which they might attain
A nobler life, with Reason harmonized.
But now, unbid, they pass on divers paths
Each his own way, yet knowing not the truth,—
Some in unlovely striving for renown,
Some bent on lawless gains, on pleasure some,
Working their own undoing, self-deceived.
O Thou most bounteous God that sittest throned
In clouds, the Lord of lightning, save mankind
From grievous ignorance!
Oh, scatter it
Far from their souls, and grant them to achieve
True knowledge, on whose might Thou dost rely
To govern all the world in righteousness;
Here is an alternate version of Hymn to Zeus (translated by Frederick C. Grant), one which I particularly like.
Therefore: in this light, we can read Seneca's Letter 60 and understand why he calls prayers for indifferents "harmful." If others were to pray for me, then I would hope they pray to grant me more wisdom, courage, temperance and the ability to grant justice where it concerns me, but not that I get a promotion or greater wealth. But we were born in a polluted world that desires indifferents.
Do you still desire what your nurse, your guardian, or your mother, have prayed for in your behalf? Do you not yet understand what evil they prayed for? Alas, how hostile to us are the wishes of our own folk! And they are all the more hostile in proportion as they are more completely fulfilled. It is no surprise to me, at my age, that nothing but evil attends us from our early youth; for we have grown up amid the curses invoked by our parents.
He then gets into humans' boundless desires. We simply can't or won't check our appetites. Yet we continue to demand of the gods for more.
How long shall we go on making demands upon the gods, as if we were still unable to support ourselves?
He notes that bulls and elephants need just a bit of land to live on and they do just fine. But humans comb the world over for food and the delve into the sea and cast up big stores of grain.
Man, however, draws sustenance both from the earth and from the sea. What, then? Did nature give us bellies so insatiable, when she gave us these puny bodies, that we should outdo the hugest and most voracious animals in greed? Not at all. How small is the amount which will satisfy nature? A very little will send her away contented.
Seneca seems to advocate for a simple life; one that is not dissimilar to Henry David Thoreau when he lived in the woods. I'm not blind to the needs of people. Much of the world still faces hunger in the year 2021 and we are still susceptible to famines. I'm not advocating everyone live like Thoreau, but I think Seneca has a point and we can learn to live rationally and minimally without impacting the environment. We can make mindful and informed decisions and change our behaviors.
Seneca quips about our desire for excess:
It is not the natural hunger of our bellies that costs us dear, but our solicitous cravings. Therefore those who, as Sallust puts it, "hearken to their bellies," should be numbered among the animals, and not among men
He talks of food, but I think cravings for all kinds of indifferents applies - cravings for fame, recognition and wealth.
The Stoic aims to help - this is his social duty.
He really lives who is made use of by many; he really lives who makes use of himself.
While others, who do nothing but collect stuff are not really living and we might as well inscribe an epitaph on their fireplace mantel instead of their gravestone.
Those men, however, who creep into a hole and grow torpid are no better off in their homes than if they were in their tombs. Right there on the marble lintel of the house of such a man you may inscribe his name, for he has died before he is dead.