When I first hear of this it was when we had our Rottie Dancer.
We took her to Obedience Training and about half way through her first year there we were moved to a new class. And we got a new instructor. Who is a wealth of knowledge about dogs.
He asked if any of us had ever massaged our dogs. To which I think was a no by everyone.
He went on to tell us that it is a good way to relax your dog after they have got use to you doing it. But there was a better reason why we should do it to our dogs at lease onec a week.
It allows us to feel our dogs for any lumps or bumps they may get and this way you will know what is feeling right or not.
So this is a relaxer for your dog after a while plus you get to knw what lump or bump has shown up by touch and you can then if you decide to go to the vet to inquire about it.
I found this is so true about the relaxing side. Though we never found any lumps at all so this was good.