The “letter” is actually a satire, or harangue, against idols and idolatry. Bruce M. Metzger suggests “one might perhaps characterize it as an impassioned sermon which is based on a verse from the canonical Book of Jeremiah.” That verse is Jer 10:11, the only verse in the entire book written in Aramaic.

Tell them this: “These gods, who did not make the heavens and the earth, will perish from the earth and from under the heavens.”
— Jeremiah 10:11

The work was written with a serious practical purpose: to instruct the Jews not to worship the gods of the Babylonians, but to worship only the Lord. As Gifford puts it, “the writer is evidently making an earnest appeal to persons actually living in the midst of heathenism, and needing to be warned and encouraged against temptations to apostasy.” The author warned the Hebrew exiles that they were to remain in captivity for seven generations, and that during that time they would see the worship paid to idols.