I’m a vegetarian (because of they way animals are treated.) Please read my whole answer before ranking it. (Please don't thumbs down it for being long)
Meat eaters eat meat because...
It is a good source of natural proteins, irons,zinc, and vitamins.
Humans have been omnivores since the beginning of time. -It's somewhat in our nature to eat other animals.
It's also what we've been taught.
Some meat eaters might eat meat because "Meat is tasty" - fair enough a lot of people enjoy the taste of meat - but there are plenty of delicious alternatives.
Some meats are very healthy and others aren't. -
It turns out that a diet excessively rich in red and processed meats is deadly. 
While most meats are a great source of natural proteins, irons, and vitamins, many meats are often high in sodium, fat, and cholesterol. Farmed animals can contain up to 50% saturated fat in their bodies.  (Why you should eat certain meats in moderation)
(pinkhamster99 (nWo) - I visited your first site and this is what I found :
Some types of meat are high in fat, particularly saturated fat, which can raise cholesterol levels. Having high cholesterol increases the chances of developing heart disease) 
As with any food, over consumption is unhealthy. Hence, eating to much meat can lead to health complications. (Again, if eaten in moderation meat is healthy!)
More than 70% of Americans include too much meat and fat in the diets and too few vegetables, according to cancer researchers. A recent survey showed 72% of Americans center their meals on animal fats and do not eat enough vegetables that could prevent cancer, and other health issues.  A survey showed that only 27 % were eating the recommended proportion of plant food to animal food.  The U.S. Department of Agriculture, the American Heart Association, the American Cancer Society and many other groups routinely advise Americans to eat a plant-based diet, adding only a little meat and low-fat dairy products.  The world health organization recommends a diet low in saturated fat, sugar, salt & with plenty of fiber - exactly what you get on a vegan/vegetarian diet :) 
One study, whose participants were aged 50 to 71, found that 11% and 16% of deaths in men and women, respectively, would have been less likely with diminished red meat intake. 
By reducing or eliminating your intake of red or processed meats, you’re also decreasing your risk of:
Heat disease – (Heart disease is the biggest killer in the UK accounting for 50% of deaths.) 
Developing/ Dying from diabetes – ( Vegetarians have a 50% reduced risk of dying from diabetes) 
High cholesterol and high blood pressure (both are associated with heart attacks, strokes, and kidney failure ) 
Developing Cancer (Vegetarians have a 40% reduced level of cancer than the general population thought to be because they have a higher intake of vitamins A,C & E) 
Premature death 
Developing Alzheimer’s disease and Parkinsons disease 
If Americans reduced their meat consumption by 10% it would free 12,000,000 tons of grain. 30,000,000 people die of starvation every year and these grains would significantly reduce this number. 
In response to your second question, just realize that there are many other ways we use animals. For example, back before cars were invented, we used horses and dogs for transportation.
Many animals can assist those with disabilities.-
One of the most important work-dogs is the "seeing eye" dog, who guides blind people and performs many tasks for which eyesight is necessary. Capuchin monkeys are being trained to live with people who are paralyzed, and to perform certain tasks for them on spoken commands.  Another important work-dog is the "K-9" dog that is trained to assist policemen and military people. 
We also use animals for companionship.