A company asked me to do a video for a reverb plug-in they were introducing, and I wanted to demo it with a variety of instruments. Drums? It's easy to find good acoustic drum loops. Guitar? I play guitar. Vocals? The company provided me with a vocal I could use. But I really wanted an acoustic piano piece, like something from Debussy, to demo the hall reverb sound. Spoiler alert: Not only is Claude Debussy dead, I don't play like him.
However, the website http://www.yamahaden.com/midi-files has thousands of public domain classical music files in Yamaha Disklavier format, but also as Standard MIDI Files. I downloaded three sets of classical music files, auditioned them through the Waves Grand Rhapsody piano, and found the perfect Debussy piece for the demo.
There are many public-domain, royalty-free standard MIDI files on the internet.
Not only that, but the performance and sound were extremely convincing—it sounded like I had gone to great expense to hire and mic an accomplished piano player. The company was happy, so once again—MIDI to the rescue.