The strong word choice by the author evokes strong feelings from readers, which shapes their perception that America needs to change drastically so that it can truly have freedom and equality.

The author's choice to end the poem this way is similar to a call to action in the sense that:

A call to action tells the audience what they should do. Likewise, Hughes tells readers that they must redeem America and its people, that they must make America what it was supposed to be.

"Let America be America Again" is a poem by Langston Hughes (1901-1967), American poet and social activist who was the leader of the Harlem Renaissance movement.
The last stanza of the poem, lines 80-86, uses very strong word choice. Words such as "ruin", "death", "stealth" and "lies", among others, convey the idea that America is rotten.
What the author wants is to evoke a strong feeling from readers. He wants them to want to change America.
In a sense, the final stanza is a call to action. The author invites the audience to do something to change those awful things about the country.
Hughes calls people to redeem America - its lands, rivers, mountains, mines, plants. By acting, people can make America the country of freedom and equality it should be.