My understanding is that it helps to dampen the bounce better from both directions......
It prevents axle hop.
I found this >> https://www.jalopyjournal.com/forum/threads/staggered-rear-shocks.983829/ and the consensus seems to be it is to inhibit axle 'wrap' - which I take to be its tendency to spin around its own axis. Hop being a by-product of that - I think we have the answer!