Why do you use MIDI files instead of MP3/WMA files?
(OR)
What are the differences between MIDI files and MP3/WMA files?

The short answer to both questions is "bandwidth". MIDI files consist of data that instructs a sound card what type of pre-stored or pre-loaded sound (like piano) to use, how loud and long to play each note, pedaling, and other special effects. Since the only data included is an instruction set and notes only as they are needed, and no sound is actually contained in the file, MIDI file sizes are relatively minute. MP3 or WMA files consist of a compressed version of recorded sound, which even with compression makes them relatively large in comparison to MIDI files. MP3 and WMA files require substantially more server space and download time (both facets of bandwidth). The advantage of such sound files is that there is a consistent rendition of the recording, whereas MIDI files are at the mercy of the sounds stored sound card or the browser MIDI plug-in.

By using MIDI files my server space needs are greatly reduced, your uploads are nearly instantaneous, and my piano sound recordings from my CDs cannot be pirated, an alarmingly common practice in the internet society that has financial impact on artists. That is also why I can provide you with hundreds of tunes without the need to shell out excessive personal resources on additional server space, and therefore can make them accessible to the public for personal enjoyment and educational purposes. That makes the trade-off worthwhile to all. The MIDI files are still protected due to the recording methodology I use, making it difficult to create accurate printed renditions or recordings that duplicate what I have on CD or Cassette. It allows the public maximum access along with incentive to help support the site through the purchase of recorded materials.