Suppliments are always an area for debate- and yes good dog food should supply all the nutrition your dog needs but...
Some dogs have special needs especially as they age that do require a suppliment. Also all dogs can benefit from extra support for their coat and skin.
My 10 year old chin was very close to neck surgery at the tune of $4,000. plus the risk of the surgery itself. I was reccommended by a friend to try a suppliment for his bones and joints and not only did we avoid surgery but his overall function has improved greatly. Based on his improvement my vet now reccommends this to his other patients over what he previously used to prescribe.
If you want to learn about suppliments a great place to go is this link-this is where I purchase all supplies for my three chins and I have been so impressed that I joined the company so that others could benefit as well. Ther is great info on nutrition and a great video clip to watch.
You can only learn more and maybe help your dogs in the long run.
If interested visit http://nancyshealthnet.com
Best to you and your fur-kids