MIDI files are computer files which generate instructions to an instrument on how to play the piece. They tell when to start a note, when to stop it, what dynamics to use for that note, etc. The sound quality depends entirely on what instrument the are used to control: a computer, an electronic keyboard, or even a full-blown pipe organ, etc. each of which will give its own timbre and playback quality to the audio.
There should be no interference with any other files in the collection, and the MIDI files are highly useful and should be included.
Other computer programs generate files that when played, use the MIDI format to control an instrument. These are often called 'source files' and are generated principally by 'notation' programs. The differences are that MIDI files contain fixed pre-set values for such characteristics as tempo, dynamics and instrumentation, while the programs playing from 'source files' often allow extensive control over these characteristics.
These playing methods are extremely useful for rehearsal, or for playing along while learning a piece, or providing missing instruments in an ensemble, reinforcing weak players, etc, or even just hearing what the piece sounds like. They are widely used when available.
It is highly recommended that all of these formats for compositions be accommodated in IMSLP because they vastly expand the usefulness of the collection by increasing the ability to play the music in various ways and have no influence on the current files for scores or parts, or even performances. Not everyone can or will want to use them, but they are a tremendous addition for those who do use them.
Sorry, I just found out that this subject has been discussed at length in other forum discussions. I do believe the subjects are related however.