Squeezing the dogs glands doesn't really have to be done but people squeeze the dogs glands to relieve pressure and prevent infection of the anal cavities.
Most people usually have their vet squeeze the dogs glands because it's unpleasant and smelly experience to do it yourself.
If your dog begins to scoot across the floor then it's usually a good sign that your dog needs to have it's glands squeezed and relieve the fluid.
I personally wouldn't do it myself and would gladly pay a vet to squeeze the dogs glands for me.