While this seems like a question that any recent grad or seasoned veteran will field during a typical interview (and hopefully they prepared for it), it's actually a great question to ask yourself. After all, the answer to it is much more important to you than the person who has posed it.
The reason you should have a five year plan is fundamental; to set up of series of steps and conditions that will lead to your personal success and happiness. For example, if you are in a job now that you enjoy but doesn't quite elicit a sense of passion or excitement for the industry, product or service you support, then you need to start thinking about a career change. Getting yourself ready for that leap will require an investment of time and effort outside the work week, and may even include additional training, education and a geographic move. Giving yourself five years to make that transition may be just enough time to get your ducks in a row...that is, if all goes according to plan.
Life has a wonderful way of reminding us who is steering the ship. It may be necessary to step away from your professional helm to address other needs such as starting a family, supporting a spouse's career decisions, or even taking care of a sick child or aging parents. These things happen and they can absolutely throw us off course. But what you don't want to be is "rudderless". You want to know what your shooting for, and why. There are a myriad of reasons, but primarily because you don't want to find yourself ten years from now wondering why you stayed in a job, company or industry that doesn't resonate with you any longer.
What typically happens to someone in mid-career crisis is the realization that they've invested too much time in a dead-end job or boring industry. However, their earnings have grown substantially with tenure and experience, as has their style of living. They soon realize it doesn't make good financial sense to leave, but it doesn't make good personal or professional sense to stay either. Never mind the sneaking suspision they're missing out on something incredibly fun.
When you rank the demands of your lifestyle over the demands of your life, you make compromises that are not easily resolved. And the hole gets deeper the longer you stay. That's not to say you can't find a comparable position with comparable pay in another industry. However, making the business case to a hiring manager to choose you over a more experienced candidate is a tough sell. Twice as true in a poor job market when qualified candidates are aplenty.
You need a plan, plain and simple. It doesn't have to be elaborate, but it should be something you're committed to for your own sake. And, it's not something you need to share during an interview either, especially if it has nothing to do with the job or company you're applying for today. If it does, that's great...enjoy the ride! If it doesn't, then offer an honest but less detailed plan that applies to the new position, in an ideal circumstance, that includes job security and longevity.
Whatever your answer, the bottom line is no one knows precisely where they will be in five years, including your potential new boss, because life has a way of making other plans for them, too. Having a plan can take a lot of anxiety out of the future, particularly when you're not sure whether you'll really enjoy your chosen career or potential employer.
Next time a hiring manager asks you what your five year plan is, maybe you should ask them the same question.