Typically, when someone posts a song in a ZIP file it means that they are providing other materials in addition to the *.sng file itself. If we look at one of @persists’s posts (which are pretty comprehensive)
he lists what’s included. The usual things are: the midi source file(s), alternate version(s) of the song and lyric / chord / tab cheat sheets. He also mentions compatible drum kits.
The midi files are included as so someone can use it to create their own version if they want to.
Not all authors list the contents of the attachments but it’s usually fairly clear what’s what when you import the *.sng into BBM.