Classic MIDI instruments, synthesizers and drum machines can be connected to computer music production studios and software with a MIDI interface. It will convert your MIDI signal to a digital MIDI signal which can be transferred to computer via USB connection for the modern recording studio set-up.
Working just like a USB audio interface, which transfers an audio signal to USB, a MIDI interface ensures a fast, clean and direct transfer of MIDI signals to USB. So you can use a MIDI keyboard to control MIDI functionality in music production software. Or more significantly, you can use music production software to send MIDI signals to your classic MIDI only synthesizer or drum machine.