Many vegies claim that 'we can get all the vitamins, protein etc from vegetables and nuts, so there's no need to eat meat'. In part that's true, but our digestive system can't extract and convert many of these as efficiently as it can from meat.
I always look at it this way - On the African savanah, all the herbivores, Wildebeest, Buffalo, Atelope etc spend the vast majority of their time eating grass and other vegetation. While carnivores such as Lions, Leopards and wild Dogs are either hunting, sleeping or on the lookout for a mate...... Bit of a no brainer really!
I read somewhere