This is a classic question in biology, and a controvertial one. That main thing that being warm-blooded does is increase the rate of our biochemistry. This has some advantages. For instance, being able to live in very cold environments (all artic wildlife, for insance, are warm-blooded), being able to be active even when its cold, and always being able to move quickly to hunt or avoid predators. However, all this comes at a cost. We must find food to maintain our body temperature, unlike cold-blooded animals, some of which can go months without eating.
Some biologists believe that the advantages outweight the costs, and so birds and mammals, such as ourselves, evolved warm-bloodedness for this reason. Others believe that it was just a side-product of having evolved for an active lifestyle. I’m not sure which theory, if either, is correct.