Sometimes, when mutating an atom, it's desirable to know what the value before the swap happened. The existing swap! function returns the new value, so is unsuitable for this use case. Currently, the only option is to roll your own using a loop and compare-and-set!
An example of this would be where the atom contains a PersistentQueue and you want to atomically remove the head of the queue and process it: if you run (swap! a pop), you have lost the reference to the old head of the list so you can't process it.
It would be good to have a new function swap-returning-old! which returned the old value instead of the new.