It may be time for taking stock of one's career over the last year...the holidays being so quiet and all and that resolution thing looming. But some folks are just happy where they are, thanks. Company treating them great, good pay, still loving the job. So I've thought a bit about why someone might want to update their resume at this time of year anyway:
* To coincide with annual or semi-annual performance review process. Review process makes you step back and look at what you have done over the previous period of time, quantify results, etc. Sounds like the same process you would go through to write a resume. So as you are putting together your review documentation, review your resume. Does it still make sense? Have your responsibilities or results increased over the last year. While this stuff is top-of-mind, spiff up the resume. It's easier to do it now than any other time.
*Because you should always have an updated resume. OK, so you aren't looking. But what if that DREAM JOB crosses your path; the one and only job you would ever even consider leaving your current gig for. You know the one. You should always be prepared to pursue your dream job.
*Because management and organizational changes happen and it's a good tool to help your new manager understand what you do. I myself have had quite a few different managers here at Microsoft. And each time a management change happens, I start to think about how I am going to explain what I do to my manager (especially challenging now that I am in a non-traditional role). Resumes are good for that.
*Because when you need to update your resume, that's when you least want to do it. Because when you need to update your resume, you may not have access to your past metrics and reviews.
*Because now there's someone sitting in front of the fire place with time on their hands to proof-read it for you. Is that just at my house? Oh wait, that's just my dog. But you know what I mean.
Some other staffing folks may want to jump in here with some other reasons.