I gave my 5 kitties vitamins when they had runny noses and they seemed to have cleared that up. It might have been vitamins made for clearing stuffy noses. I don't remember where I got them, though, or what the brand was. They cost $14.00 at a pet store around here.
They aren't necessary if they get a good quality food. However, some vitamins and fatty acids can help with their coats, growth, digestion etc. It's a plus if they get them, but not a necessity. I really like the 3v caps ar liquid. It has omega 3 and 6 fatty acids with vitamin A and D.