Why do you have to cook heroin for injection?

You do not have to cook heroin for injection if it is 100% pure, because pure heroin dissolves in water. However, it is almost impossible to find pure heroin.
There is still controversy over whether heroin should be cooked/brought to a boil. Some maintain that doing so is more likely to kill off harmful viruses and bacteria. However, others argue that you would have to cook it for 5-15 minutes for it to have any significant effect, and that by heating smack cut with other substances it just dissolves the impurities into the rest of the solution. This can be dangerous because the cut is often harmful when injected.
It seems that even medical authorities tend to disagree about how the safest way to inject is in the real-world, non-laboratory setting. In the end it comes down to personal preference. Of course the safest choice is to smoke or snort, but injecting provides a significantly better high and is more economical, at least until tolerance develops so there will always be iv users.