Why is the body warm?
Heat is mostly generated in the body as a byproduct of metabolism
Blood can be warm because of many reasons but main one is the heat produced during metabolism in liver.
The body is warm in order to keep key reactions.
Most, not to say all, reaction in the body happens in a pretty slow rate, they are connected to metabolism. They are two ways to make reactions to happen faster, by increasing temperature, which effects all reaction, or by enzyme. Most of the reactions in the human body needs enzyme to happen, some medical conditions happen due to lack of certain enzymes.
In the two cases for increasing reaction rates, you must increase temperature, but not too much. Even the so called cold blood animal need to stay in a window of temperature.
Why is the body warm? it is warm to keep key reaction happening "in time". The heat is generated by several internal processes such as thermogenesis