As to "why" they did it, I suspect marketing had a lot to do with it. Fractional rigs became popular with racing boats, such as the J-24, and suddenly lot of non-racing, or non-racing-specific, boats followed suit.
For racing boats, the advantages are pretty clear. More aerodynamic upwind. If they don't reach as well under normal sail, doesn't matter because they're going to reach with a spinnaker anyway. Downwind you have the big main to complement and stabilize the spinnaker. The rig, with the flexible tall mast, is more responsive to fine tuning, can be depowered more easily w/o reefing as early.
For normal recreational sailing, non performance focused, the only advantage I see is the small headsail. Not only is it easier to tack with, it is often easy to set up as self tacking.