1.) The songs you download are mostly more correct and complete than many normal tabs in the web, and also include real notes(So rhytmn is also there), which might be useful for some keyboarder who wants to play a guitar solo or something.
2.) You can play the song in midi format and theres a red signatur at a note, when it is being played at the moment. So you can learn songs easier because often rythmn and accents are difficult to find out with a normal tab.
3.) You can also edit your own stuff, your own licks and riffs, and jam along them. This avoids that you forget some of your riffs and licks(that happened a lot to me, because sometimes a good riff is only cool, because of one detail, and you forget this.
4.) Mostly there is also the bass tab of the song included.