It's just a method to break it down into easy chunks when you are learning how to establish a constant steady approach.
A lot of civvy schools teach this method, if high or low adjust power to fix it, if fast or slow adjust pitch attitude. This is really just a way of thinking which to adjust first. If you are high you would reduce the power, but you would soon need to lower the nose or the speed will bleed off. One way of imagining this pairing of attitude and power is that your arms are linked by a piece of string, like you were wearing mittens as a kid. If you adjust one thing, e.g. reducing power, you do the opposite with the other hand, in e.g. pitching forwards.
The military tend to teach the opposite, power for speed and pitch for rate of descent, as this method works a lot better when you come to fly an instrument approach or fast jet. Either way it amounts to the same thing and is just a way of breaking it down in your mind into something more manageable.