I don't know that anyone actually prefers it, but think it is it is sometimes better for use in flexible conduits where there is vibration, like at motors, or to take strain off relay terminals.
Sometimes on long conduit runs with tighter bends it can pull easier, but not always.
I do remember reading some theory that stranded wire could carry current more efficiently than solid wire.
It had something to do with eddy currents between the individual strands, but I would have to hunt around on the web to find it again. It may have just been a bunch of balony.
