I have just switched over to grain free and my main reasoning is for the higher protein for my senior golden.
Also, he has some food allergies (corn/soybean/wheat), so I've always avoided these foods anyway, so I'm trying grain free to see if it further helps with that.
With grain free, you're going to have a much higher percentage of meats, which dogs are meant to eat and seem to thrive on (at least most dogs).