This is because instead of recording sound to a file, when using MIDI, we record the note number, duration and other parameters about the note being played. However, with MIDI we can do all of these things. The limitations of a sound file prevent us from effectively slowing it down or transposing it to new keys or seeing on a staff what notes are being played. When recording a keyboard (digital piano), we normally think of recording a sound file like an mp3 file. MIDI stands for Musical Instrument Digital Interface and it is a way for computers and keyboards to talk to one another.
