Let’s say you have the Key Editor open and you select three notes in a chord, move them a little and then move another note and all of a sudden you realize you need to move the first three note group to compensate.
No, problem. Just do it!
Now … you select the top harmony notes in a chord progression and do something with them, then you select the next harmony line by carefully selecting them and move them too and … you need to move the first group again.
OK fine …
Then you exit the part and realize you have 27 individual parts left … 
Wouldn’t it be useful if you could group the notes as you can do with parts in the Project window and have things that belong together move together?
Even better if you had the functionality borrowed from the Faders in the MixConsole so you could hold down the [ALT] key and select and move an individual note in a group without the others following.
You know you would love that, don’t you? 