You can use both independently and with the right software and soundcard, use both simultaneously to create some really nice music. MIDI reminds me of the piano rolls that scroll on player pianos. MIDI is nothing more than a series of commands that tell a synthesizer what to play and how to play it. It is not audio. That means that MIDI file sizes for even lengthy pieces of music are small.
Digital audio is another story.
When you input sound into your soundcard and it is converted to digital audio, large amounts of hard drive space are required. As for MIDI, General MIDI ensures that assigned instruments play. Digital audio sounds the same, just as a CD sounds the same on your player as it does on mine. With digital audio, if you want an oboe to play a part, instead of a flute, you have to re-record the entire track. With MIDI, you can re-assign instruments to the appropriate MIDI channel and not have to record anything new.