"September 1913" begins with the first of several rhetorical questions addressed directly to the reader ("you"). Right away, the speaker's tone is aggrieved and antagonistic: this opening question is an interrogation of readers' values and principles.
Yeats has a specific audience in mind: the Irish middle class in the early 20th century, scathingly described here as "sens[ible]" folks who have no "need" apart from adding up small change and counting the rosary until they die. (In the Catholic tradition, a string of beads is sometimes used to help keep track of how many prayers a person has said.)
It's clear from the language that the speaker looks down on these activities. They describe the till, or cash register, as "greasy," suggesting that maybe there's something ill-gotten about the money that's going inside. It's not a lot of money, either, implying that these people are both greedy and petty. The diacope of the phrase "prayer to shivering prayer," meanwhile, suggests that their religious piety is done thoughtlessly, out of dogged obligation rather than genuine faith. The readers are piling up their prayers much like they "add" coins to the "greasy till," as though trying to bank some goodwill with God. The word "shivering" further suggests that these prayers are weak, ineffective, or sickly.
Making money and putting on empty displays of piety don't count for much, as far as the speaker is concerned. In fact, they're life-sapping: the metaphor in lines 4-5 implies that a focus on material things and Christian salvation actually results in the vital "marrow" of life being dried out, leaving nothing but a hollow, brittle bone behind.
The sounds of the poem help to build its acerbic tone. For example, note how the plosive /p/ alliteration of "halfpence to the pence [...] prayer to shivering prayer" help to convey the speaker's bitter distaste.
Most of the poem also follows perfect iambic tetrameter (four-beat lines that follow the rhythm da-DUM, da-DUM), lending it a precise, snappy rhythm. The opening line, however, features a variation in the meter. The second foot here is a spondee (two stressed beats in a row). This combines with the caesura, or pause, after "you" to unsettle the reader:
What need | you, be- | ing come | to sense
The extra emphasis on "you" implicates the audience from square one. This isn't an abstract second-person address. Again, the speaker has a particular person, or group of people, in mind.