Some doctors mix the hyaluronic acid fillers with saline solution, what is the benefit of that?

I'm not a fan of mixing saline with hyaluronic acid based fillers. I do know of several of my colleagues that do this. They are trying to make the product a different consistency whether it be to make it easier to inject or better for a particular area of the face. In my opinion, the leading manufacturers of these products, Galderma and Allergan, both have a broad portfolio of filler products and I do not think there is any need to change the products prior to injection. We do not know how mixing the products with saline changed longevity or effectiveness. Mixing the filler prior to injection may also increase risk of contaminating the filler.  Again, this is my opinion.