All
clubs look round. In actuality, they're oval shaped. Since they're actually oval shaped, one side is stronger than another (the ends of the oval). A spined shaft has the strong ends aligned. One alignment combats toe droop (spine at 12 o'clock). Another adds distance (spine away from target) and yet another adds control (spine towards target).
A club will bend in the same way each time it's swung. Properly done, a spined club will come back to the same point as you set it up in. An unspined club may waver off line a little bit - either to the toe, or heel, or high, or low.
Spined
clubs make for a repeatable club response, so that it plays the same way swing-to-swing, whether you're 'stepping on it' or 'easing off', the clubhead returns to the same position time and again.