I've been using some precious time to do this. I'm finding it helps in many areas:
- It gives me a better understanding of the harmony because the new keys are harder to figure out without this understanding.
- It definitely helps with hearing intervals, especially leaps.
- It creates a better understanding of all keys.
- It helps to play in keys that don't always get played in and breaks the tactile memory and makes the player work harder to overcome this.
- It's great for technique and fingering issues.
- It is good for the understanding of voice leading.
- It helps with hearing and the understanding of tonality and all twelve tonal centers.
- It helps in the development of piano texture-creation in the new keys which will influence the texture and understanding upon the return to the original key. I always come back to the original key refreshed.
- It helps tremendously with improvisation and line creation. Now I can better improvise in these keys and others (I say to myself).
- It mainly benefits the inner ear and solidifies the sense of a particular tonality.
I'll follow this article with another featuring one of my favorite and most useful aspects of 7th chord-tones substitution and the pathways that are present when one or more of the chord tone leads to an adjacent chord extension tone—it can contribute to the solo line concept as well.