Something was needed to solve the problem of each company using a proprietary scheme to trigger keyboards from a master. MIDI does not transmit sound. When a note is released a new message is sent telling the receiving device to stop the note. If a MIDI musician connects many instruments together and assigns them all to the same MIDI channel a thick layer of all of the sounds would be heard whenever the master keyboard is played. M anufacturers soon recognized that slaves didn't need to have keyboards since they are not actually touched in performance.
Most new instruments are available with or without a keyboard.
Sound modules usually cost approx. After buying one or two keyboards most MIDI musicians opt for sound modules to complete their MIDI systems. D rum machines are sound modules loaded predominantly with percussion timbres. These can be played from external controllers but are most commonly played by pads built-in to the device. Sequencers allow the musician to record, edit and playback MIDI information.