If we travel space, we will never find a planet similar to Earth where there is only vegetation and no animals. Vegetation breathes carbon dioxide and exhales oxygen. Without animals to produce carbon dioxide, eventually the plants would use up all CO2 and become extinct. Plants need animals as much as animals need plants.
So, imagine for a moment that you are the "intelligent design" behind all of this, here on Earth. What would you do to create a balance?
First, you would create plant life, without which a planet would remain forever barren. But unless you do something to prevent excessive proliferation, plants would, as described earlier, use up all the CO2 and die off. So, as a thinking person, you introduce herbivores - plant-eating animals. This not only prevents over-proliferation of plants, but the animals also convert oxygen to CO2, so the plants will have an endless supply. But now you have a new problem - unless you figure a way to prevent over-proliferation of herbivores, they will become so plentiful that they would consume all vegetation. Vegetation becomes extinct, and so do the herbivores that need vegeation for food.
What to do?
You introduce carnivores - meat eaters. Now you have plant life, in balance and under control; herbivores in balance and under control, and things are balancing. But what keeps carnivores under control? Frankly, it is availability of food.
If too many carnivores are born, they consume too many herbivores. As herbivores become less readily available, the carnivores start to die off from starvation. This creates the "cycle" often seen in nature - every few years there is an abundance of rabbits. This leads to an abundance of predators, which leads to a shortage of rabbits, which leads to a shortage of predators. The new shortgage of predators allows more rabbits to mature and reproduce, causing a new glut of rabbits, and the cycle goes on.
So, as intelligent designer, you would create plants, plant-eaters and meat eaters, to keep everything in balance.
Man, by design, is an omnivore - we are designed to eat both plants and animals. This is easy to see just by looking in the mirror and smiling. Nature gives each animal the teeth it needs for the diet it is supposed to have. Man has both kinds of teeth - molars, as well as canines. Mother Nature (or God) says we are supposed to consume a diet of both meat and vegeatables.
Vegetarians say we should eliminate meat from our diets. Assuming Mankind could do so and still achieve optimal health, what about the unintended consequences? If we no longer consume meat several really bad things would happen, some of which would cause our own extinction.
First, millions of people would loose their jobs - anyone who raises, processes or sells any meat items. Next, vegetables, themselves would suffer, as roughly 90% of real fertilizer would disappear - livestock currently produces the fertilizer necessary to grow vegeatables. No livestock means no fertilizer, which means no veggies.
And we also face the dilemma of a serious reduction of CO2. Livestock produces much of the CO2 that plants require in order to breathe. If plants start to die off due to a lack of CO2, WE start to die off, because it is those plants that produce the oxygen we need, as well as the food we require.
So, here is the short take why Man should consume both meat and plants - SURVIVAL. We cannot survive if nature loses its balance, which is what would happen if we stop consuming meat.
As a final note, it is precisely nature's well-thought-out balance that helps proves the existence of "intelligent design". Could a dummy figure all that out? If not, then what are the odds that it all happened by "chance"?