Most people are familiar with the phrase, "survival of the fittest". Darwin's theory of evolution uses this phrase as a foundation, for it explains how organisms evolve. Creatures that are able to survive long enough to reproduce will have genes that allow them to do so, and in reproducing these beneficial genes are passed on. Creatures that have genes that make them less likely to survive are unable to pass on these genes, and so the unbeneficial genes are removed from the gene pool. Survival of the fittest.
For evolution to work for the benefit of a species, bad genes must be removed from the gene pool. Enter the predators. Predators are a means of removing unwanted characteristics from herbivorous animals like zebra and deer.
Take the example of the gazelle. The gazelle is an animal that escapes its predators using its speed. It has for thin but strong legs that enable it to reach speeds of around 50 mph to escape from predators like lions and hyenas. These long legs have proven to be evolutionarily beneficial. Now imagine a gazelle was born that, through a genetic mutation, had three legs. This is a very unbeneficial gene that will be removed from the gene pool because the gazelle won't get the chance to reproduce. A three legged gazelle will get eaten. A rhino without a horn will get eaten. A wingless owl will get eaten.
The role of predators in terms of herbivores is to purge the species of unwanted genes. However, it is not just herbivores that evolve, for in order for the balance between the number of predators and prey to be equal, the two must evolve together. Too many predators will result in too little herbivores, and the reverse is true if there are too few predators.
So how do predators evolve? For this group, the fittest animals are those that can get their food. Take the example of the lion. Its color allows it to blend in with the savanna, allowing it to creep up on its prey. Now imagine a lion was born with a gene that turned it blue (I'm not suggesting this will happen, this is a gross exaggeration of a possible genetic mutation). Its prey would see it coming a mile away, and the lion would never catch anything, dieing before it is able to reproduce.
Predators have the evolutionary responsibility of making sure that herbivores are kept in check. Without predators, evolution would not have travelled in a beneficial direction.