this seems more or less orthogonal to how necessary a job is. really what this question tests is time sensitivity. if you're the only engineer working on a project that needs to be done within a year, you could conceivably go on leave any month other than the last one or two of the year. if you're a fast food worker and you want to take a month off, they will either need to hire someone else entirely or make you carefully coordinate getting your shifts covered by co-workers. I don't think this implies the fast food employee is more necessary than the engineer.