In most cases there is no need to add supplements to a well-planned raw diet.
Some pets do well on a course of probiotics, or fish oils. In most cases, this supplementation is a temporary measure to control inflammation, and populate the gut with beneficial microbes in order to assist immune function.
However some cats and dogs have a very compromised immune system, and a major microbial imbalance in their gut. These are the animals that will suffer deficiency even on a really good raw diet, or a processed one. Unless the gut health of these animals can be significantly improved, they may benefit from continuous supplementation.