Actually, this is a pretty good reason, and it's straight from the source (creator of S, the predecessor of R): https://stackoverflow.com/a/51548423/139922

> When you type a long expression only to remember at the end that it would be a good idea to save the result, a right-hand arrow allows you to perform an assignment without retyping the line.

(I don't do this personally, though. I just run the expression and then my next command is `x <- .Last.value`)