Experience with the domain doesn't need to have much to do with it- most of what I mean by "problems that might happen" are just pure software maintenance issue

Experience with the domain doesn't need to have much to do with it- most of what I mean by "problems that might happen" are just pure software maintenance issues. Perhaps your new function gets a value that you know can't possibly be null; you don't need to write a check for null, then, since it will still always work. But a good software developer might feel untidy leaving unchecked data into code that might be later reused by null-value-using colleagues, and will take the two seconds to write the check. Furthermore, if he doesn't write the check and this turns out to be wrong, "just throw it away" isn't an immediate option; finding and fixing the error caused could take orders of magnitude more time.