Having looked at calorimetry as a way of measuring heat, we now need to look at the bigger question: Why do we want to measure heat?
We answered that to some extent before, when we discussed determining the calorie content of food.
The more complete answer is simply that reactions absorb and give off heat and that the only way we can measure that heat is to have that heat given to, or taken from, something else. So, if a reaction is run in a cup of water, and the water gets warmer, then we know that the reaction must have given off the amount of heat that the water absorbed.
So, the next question then is why do reactions give off or take in heat at all?