What are reasons to update your website and CV frequently, when not on the job market?
A CV in academia is useful to you and others in many different contexts.  Applying for jobs is a very small component of that.
CVs are useful to you and others. Keeping an academic CV up-to-date will often make your life easier, other people's lives easier, and may also open up opportunities for you. 

Here are a few ways that having an up-to-date CV is helpful:


Communicating achievement to others: The decision of others in relation to you may be influenced by perceptions of your experience and achievements. Your CV documents this (although google scholar profiles and personal websites can also serve this purpose). There are many examples that could be given: (a) another academic is working out whether to invite you onto a paper and wants to understand your expertise, (b) potential phd students are looking for a supervisor and are trying to understand whether you've supervised before or how much and how well you are publishing, (c) your colleagues at your own university are trying to understand what you do.
Grants: When you apply for grants, you will need to incorporate information about your track-record and achievements. Your CV can be an up-to-date source of this information. Furthermore, you might be part of a grant that others are leading. The grant will often require a lot of information about the investigators on the grant. These other people may be able to fill out relevant sections directly from your CV.
Annual performance review: If you have an annual performance review, it is often helpful to share your current CV.
Internal promotion: Systems for promotions and levels vary throughout the world. In Australia/NZ/UK you typically have levels A to E. If you want to move up a level, you need to document your achievements. Having an up-to-date CV can help.
Course accreditations and related matters: In some cases, your department or university might be seeking accreditation as an educational provider in a particular area. These often require documenting number of staff, educational attainment of staff, evidence of research active status.
Annual or other reviews of programs: In addition to performance reviews, programs, larger research groups, and departments often compile documents reviewing annual (or other timeframes) achievements (e.g., publications, grant funding, outreach, etc.). Administrative people can go to your CV to get this information. Or it may be that you open up your own CV and copy and paste a bunch of things.