What is the difference

 If he's doing well on what he's eating, and he's had problems in the past, I'd stick with it.

Seniors gain weight easier, so watch his weight. They can use a little extra protein, so you could add some fresh meat or canned food to his meals, to give him a little boost. 

I'm sticking with what we've got. I was just wondering IF and when and why it could or should be something to consider...

 I think you need to consider it when your dog just doesn't seem to be doing as well on his regular food.  Gaining weight, having other issues. 

  Senior foods have glucosamine and chondroitin added to help prevent arthritis, but it's usually not enough to be therapeutic. Jessie's nine and I add a human joint supplement to her food. Some companies such as Eagle Pack, Royal Canin, and Purina make senior foods that are higher in protein (26 to 28%) and lower in fat to help older dogs maintain lean muscle mass; both Royal Canin and Eukanuba have done research that shows senior dogs maintain better muscle mass when fed more protein. I agree with the others; if your dog's doing well with what you're feeding now, there's no need to change.

I agree with everyone else if your dog is doing well stay where you are. 

That said, I'm probably in the minority but I do like feeding puppies and older dogs food specific to their age.  In my mind those formulas might have the extra boost they need at those special times in their life. 

you have to be careful with senior foods- many of them actually reduce the amount of protein compared to regular food for some reason, which is the opposite of what your dog needs.

     I have one who is about to turn 8yrs, and don't plan on changing him to a senior diet anytime soon. The only reason I would go with one is if the dog had special needs that a senior diet could more readily address ... for instance, a predisposition to kidney problems would be better maintained on a lower protein food with adjusted calcium:phosphorous ratios. Or a dog whose metabolism slowed down in old age might do well with the higher protein/lower fat/reduced kcal senior foods. Otherwise, if it ain't broke ...