In Welty's "A Worn Path," Phoenix is so poor that a nickel, to her, is a great amount of money. She gets excited when she sees the nickel fall out of the man's pocket. This puts her in a precarious position. First, we can assume she is hoping that the man does not realize he has lost the nickel. Then, after she picks it up, we can assume she hopes that he doesn't realize she has his nickel. She feels apprehension because she doesn't want to get caught. She also, though, as she says, feels shame for picking up the man's nickel:
God watching me the whole time. I come to stealing.
This is a sick, distorted world Phoenix lives in. It is a Southern Gothic world. Tension is heightened when the man points his rifle at her face. The reader, like Phoenix, doesn't know if he points the gun at her because he knows she stole his nickel, or for some other reason. As it turns out, he points the gun at her just for kicks.
This quote and the entire situation is indicative of the world Phoenix lives in.
I do not think that the quotation itself has that much importance, but the nickel does.
Phoenix will, in a little while, pick up the nickel. In a sense, her taking the nickel gives her a small amount of revenge on the white man. He treats her very badly. He makes fun of black people, saying they can't resist Santa Claus. He threatens to kill her. She gets some small revenge by taking the nickel.
Seeing the nickel also shows how venal and dishonest he is when he tells her he has no money to give her.
I'd give you a dime if I had any money with me.
Finally, it allows us to see Phoenix being somewhat honest -- admitting she has just done the man some unspecified wrong:
"No, sir, I seen plenty go off closer by, in my day, and for less than what I done," she said, holding utterly still.
Thus, the nickel allows Welty to comment on the way that whites treat blacks and, in this case, the relative morality of the white man and the black woman.