Well, less preservatives lessens the chances for some types of cancers. Better nutrition means better coat, more energy, and many other things to numerous to list.
I will tell you a little story to illustrate the difference in my senior girl.
Last August I took her to a fund raising picnic to raise money for rottweiler rescue. The running joke of the day was "Sandi, can you please control your dog." You see, I put her in a down stay in the aisle between the picnic tables and the chairs (right in the way of everyone trying to get by of course). She promptly laid on her side and went to sleep. She spent almost the entire day in that same position, with everyone stepping over her to get by. She had no interest in anything that didn't involve food.
A couple of weeks ago, we went to the picnic this year. (It is an annual thing.) Since I helped organize this years picnic, I was busy running all over the place. Chase did not sleep a wink this year. She was up, wandering around, usually a few paces behind me. This year she even went wading in the pond! And the compliments! Her coat looks great. She has lost so much weight. She has so much more energy.