Why do people eat meat?
There are many reasons for why people eat meat. Some of them are to be discussed here and some of them have nothing to do with biology.

Reasons that have nothing to do with biology

There are many reasons that are not to be discussed on Biology.SE. These include cultural reasons, the reasons related to taste and appreciation, farming and availability, price and economy and of course for reason related to political ecology (incl. water and energy consumption of meat production).

Reasons that has to do with biology / medicine

We can talk about nutritional value though. According to a number of websites (sorry nothing peer reviewed; monitor.co.uk; rd.com; sparkpeople.com), people not eating meat may suffer from lack of 


protein
iron
zinc
calcium
vitamin D
vitamin B12
omega–3 fatty acids


It is not impossible to get all of these nutrients without meats. However, when becoming vegetarian, some people need to be careful and rethink their diet to ensure not to suffer from the lack of these nutrients.