As has been stated, what you can do with a midi is limited pretty much only by the board you're using to play it ... I have found some free midis on the net that are pretty darn good, and some that I wonder why anyone took the time to put it there... Pick a song you want, let's say 'Misty' ... do a search on "Misty midi" and see what happens ... or do a search on 'jazz midis' or whatever you're interested in ... There are also a number of 'pro' midi sites where you can purchase the midi of your choice...
I use a technics kn6000 and I've lengthened, shortened, changed voices, etc. and then saved the midi as I want it ... using the excellent sounds of the kn6....
As creative as making your own song files using the styles on your board? .. perhaps not, but for certain tunes they can be a lifesaver.... or gig (job ) saver ...
t.