1) In theory, all blanks have a center spine, and a casting rod, with guides above the rod, will twist under load. Under heavy loads (like a 5 pound fish and 10 pounds of salad) a rod with lots of twist will normally break within 6 inches of the tip.
Imagine taking a small pvc pipe, tying a rope to it, winding the rope around the pipe, tying a heavy weight to the other end, and picking the weight up by the pipe and rope. The pipe is going to try to twist until there are no more wraps around the pipe and the weight hangs straight below it.
A spinning rod, with the guides below the blank, imparts no twist on the rod.
A spiral wrapped rod gives you the power of bait casting reels combined with the strength of a spinning rod, in one combo.
That's the theory.
2) The real reason is I just think they are cool and wanted to build one.