A lot of you gamers might be hardcore players, and don't pay that much of attention to detail and sound. But try and play your favorite game without the sound and you'll notice that a big part of the game is missing. A part that you in general don't pay that much of attention to, untill it's gone. You can be up to your knees involved in a gaming experience to the point when you notice something that pulls you out of the game. i.e, Gaps and flaws in the design. Things that distract you from enjoying the game to it's full extent. It might be a bad sampled sound or a sound that just don't seems to fit in. Sounds in general puts you "in" the game, so this is the biggest factor in keeping your gaming experience at top. So the biggest reason for an enhancement would be in replacing all these sounds making a great game, hopefully even better.
Graphics on the other hand is the visual design and trademark of the game. "Replacing" that would be like changing what the game really is about. So if a enhancement features some kind of graphics it's just that,.. Enhanced versions of the original thing, making it better or more usefull in some way.
All the enhancements are really a tribute to the original game. Games that i enjoy playing and for some reason found small issues that in my mind should have been "fixed" or changed prior to release. Then again, i have no idea about what struggles a gaming company needs to go thgough in order to get a game out there. But im pretty sure it's all about deadlines and budgets. So hats off to all you creating the games we all love to play.