Probiotics (good bacteria) can benefit your cat in a variety of different ways. When probiotics are given to a cat on a regular basis they can help in the prevention and treatment of certain problems relevant to felines. Just a few examples of issues that probiotics can benefit are diarrhea, allergies, stomach problems, stress/anxiety, immune system, etc.
While you can give your cat some types of probiotics made for humans, their gastrointestinal tract has a host of different types of bacteria that will benefit from a formula designed specifically for them (IE. Bark & Whiskers Probiotics for Pets).
