With Synthesized music on the rise and eighties music making a comeback you will undoubtedly see some vintage synthesizer or groovebox that you desire. But there is only two ways to keep them in time. If your lucky the instrument will have a tap tempo, but if it does not you are stuck with twiddling knobs and pressing buttons every time you want it all to be in time. The other way is to connect them via MIDI so there is no messing around when you need to be concentrating on the music.
Perhaps you have a sampler with an array of awesome samples you have built up over time, but you don't have a way of getting them to play without scrolling through banks of them at a time. That is where MIDI programming comes in. MIDI is short term language for Musical Instrument Digital Interface. Many mistake them for audio cables, but this is not the case. They are used to send information between each instrument in your set up to trigger certain effects or just to keep in time. The more advanced MIDI user can programme his/her entire set up so when they press play on their master MIDI controller it will trigger an entire song or composition on all of their instruments.
The MIDI lead is as essential as audio leads as part of the modern electronic musicians set up.