Yes, I've read the comments that say "MIDI, what is this 1980-something?" But as a Music Teacher/ Musician that also does theater sound and lights MIDI still has many currently used applications. Sure, MIDI began as a way to listen to music on a computer, back when a few KB was a lot of memory. However, MIDI also has the ability to clock-sync devices for synchronous playback. So if you are at your favorite band's show and the music and the lights just seem to time out perfectly, they probably do because somewhere a MIDI device (or long chain thereof) is keeping the lights and sound in synch. And this is a complex example. Even cheap DJ equipment can use MIDI singals to control lighting.
The second major area MIDI is used is in Sheet Music Creation and Playback Software. MIDI provides a background framework for playback of files in software like Finale, Sibelius, MuseScore, etc. MIDI defines the duration, volume and pitch of a note and the software uses high quality sound samples for playback. Furthermore, in the educational sphere, a teacher can write out an exercise for the student to play. The software records the student playing and sends it back to the teacher. But if the student wants to listen to the exercise played by the software, it's going to need a MIDI capable audio system to do so.
tl;dr: MIDI was once used for consumer audio distribution, however the protocol evolved in several important ways making it the backbone of virtually all comupter audio creation systems in use today. The cheesy synth sounds are (mostly) gone, but the protocol lives on behind the scenes. If you've ever been to a live musical event where computers were used in some way, odds are MIDI was the protocol keeping it together.