MIDI can be very useful. Not only is it for instruments, but also for anything that can be controlled, like lights or sequencers. I have used MIDI for a while and it used to be a bit mediocre, sounding somewhat electronic. Now, I am very pleased to find that the sound of the MIDI is better than ever. I can almost feel like it's a real instrument. If not for the sound that my keys make when I play them, it would feel like a true piano.
Furthermore, there is so much more to the picture. MIDI can be used to trigger loops or drums or anything really. Plus also pads and knobs can be connected to virtual instruments. MIDI is even useful for live performance. You can make a patch for whatever sound you like, and play it. It's like having any sound literally at your fingertips.