Usually, because they help narrow down the sources of error. If you've tested your code so you're sure the various bits work as intended, any remaining somewhat

Usually, because they help narrow down the sources of error. If you've tested your code so you're sure the various bits work as intended, any remaining somewhat crazy results are down to things you don't understand properly. A somewhat validated code allows you to do by computational experiments what you can't by real life experiments. Very handy if one of the common end states in the experiments is "death".