If human A is a lot better than human B, then A should be able to beat B without having to find B's least favourite positions. However, in computer play, something different is happening: they're getting so good that they're able to get draws even against stronger players. It seems likely to me that computers will be able to become almost completely unbeatable. Until this happens, though, they are beatable, and in order to beat them, one must find types of positions where they might make a big enough mistake to be beaten.
Once the strong computer is 2-3 points ahead, it can revert to best play to find a reasonably quick win.