I've used SPD switch a lot when i was flying 737NG mainly on approach while G/S capture on the FMA.It deselect A/T servos without deselecting Autothrottles hence giving you manual control of the thrust but doesn't protect you from the gust,it gives you Alpha floor protection meaning that in case of underspeed A/T quickly recovers the speed for you avoiding you a stall.The rest is like Cosmo kramer explained in depth