Say, you played/improvised a sketch with a midi-piano, not using metronome, but still more or less rythmical with some rubato. Now if you want to put it to the grid it will be a big pain in the ass. If it is audio - it is really very fast and simple to grab a transient-makers and align it to the beat.
... Surprisingly enough just while writing this I discovered that you actually can select an empty region in MIDI editor without selecting any notes, and it will do the job, kind of, stretching the notes while moving the rest of them outside the selection. But still audio is much easier for warping. Could we please have each note represented in the upper lane as a transient with ability to make it a marker with the same behavior as with audio?