I don't really believe in "diets". Eat healthy, cut down on bad foods, but don't deprive yourself of anything. Moderation and exercise is much better.
It isn't a diet unless it is temporary. If it is a permanent change then people can find a lot of benefits from going Vegan or Vegetarian. I should say though that I don't notice a big change between when I had already cut out cheese from my diet to when I finally removed all other sources of dairy, eggs, and the occasional seafood from my previous diet. For some people choosing to eat smaller portions may have the bigger impact. But whatever you eat, reducing the amount of processed foods can be huge.
Some people go Vegan for reasons other than health and that is a whole separate story.
