When the rear hubs are off of the axles, the axles drop down onto the grease seals inside of the axle housings. If left in that position for several days, weeks or more, the seal will set out of round and leak. A neoprene seal with a spring should recover its shape, but a leather seal like the OEM may not recover its shape and leak.
So it is a good idea to put the hubs temporarily back onto the axles if your work project will take a few days or more.