It changes
(a b c d (1 2 |3 4) ha ha ha)
into
|(1 2 (a b c d 3 4 ha ha ha))
What is the use for that transformation?
Strictly speaking, it's used for reversing the nesting of an inner expression.
As an example:
(let [foo bar] (if a b| c))
becomes
(if a b (let [foo bar]| c))
As for use case, I've never had a practical use for it specifically, but it's a nice building block for a larger macro