Interestingly, the crossover point between the two settings for Spearfish is 45 knots. Above that speed, the low-speed setting gives a longer engagement range; above it, the high-speed setting. The existence of such an extreme high-speed setting implies that the Royal Navy was concerned about targets (i.e. submarines) moving very quickly indeed. For the Mark 48, the corresponding speed is 21 knots.
On a slightly more absurd note, if you can get a 30 degree up angle on a Spearfish and broach the surface, it might reach an altitude of 70 feet or so. That's high enough to offer a theoretical anti-helicopter capability. Totally impractical, of course. But amusing to think about.