I've actually had a professor do this to us in a class. It is a very effective way to show that you can understand what order of magnitude to expect from an an

I've actually had a professor do this to us in a class. It is a very effective way to show that you can understand what order of magnitude to expect from an answer. This means that when I go and plug the orbit of a satellite into STK, I'm already looking for an answer within a certain level of precision. While this isn't good enough for an answer to "how much stress does the bolt that holds down the payload have to be capable of withstanding" it does give you a sense of what that answer should be. This is a very useful tool for realizing when your equation is returning complete crap because you are using the wrong unit (as pointed out by tolkenadult and the Mars probe example).