What's the point of maintaining a CV after tenure?
Upon joining the club of those who've "made it," you find that there are further gradations. A tenured professor can safely breathe a sigh of relief for at least a year or two, but eventually she will notice (i) that her salary is lower than most of her tenured colleagues and (ii) that she is still being asked to document and defend her productivity across several axes: research, teaching and service. (Also, in many cases, (iii) She is increasingly pressured to apply for and get major grants. In many cases, her future salary and promotions depend on this to a large degree.)
The main tool to exhibit your recent accomplishments in research, teaching and service is...your CV, usually. In fact, "whether they want a good raise or not," most faculty are required to submit annual "activity reports" in which a CV is often a required component. (When it isn't, it usually means that we manually copy information from our CV into some form or type it into some internal webpage. So having an updated CV is more or less necessary for many faculty members.) Certainly having a carefully groomed CV is necessary when you go up for "promotion to full", apply for an endowed chair or distinguished research position, and so forth.
The bottom line: academia has bureaucratic, competitive and hierarchical aspects all the way through. Most people who have made it tenure in recent years have learned how to keep reasonably up-to-date CVs without too much trouble or heroic effort -- the CVs do not need to be rewritten from scratch to showcase their skills for some new job, usually, so it is mostly a matter of regular updating -- so honestly, this is not the worst part of the bureaucratic minutiae.
Some reasons that I've needed to have an up-to-date CV:
Applying for promotion to full professor.
Applying for a funding for a sabbatical leave visit to a research center.
Applying for grants. (Most funding agencies have very specialized formatting requirements for shortened CV's. For example, you're only allowed 2 pages for the one you submit with an NSF proposal.)
An external review of our department and its programs.
It's such a common need that I just make a point of keeping my CV up to date as new publications and other events happen.
Everybody needs a CV to help others better know them.
I am not a professor, but I think even tenured professors may maintain their CVs so as;
- As an advising professor, they need to hire or accept research (PhD or masters) students,
- As a university professor, they may hire PhD graduates in form of postdoc positions,
- As a person working in university or industry, they need collaboration;
and for many other reasons.
They need an updated and maintained CV so as people who want to collaborate with them in a research project get know them, their research interests and their previous works or at least their educational background.
Research students need to read their CV to get familiar with their research interests and know their past PhD or masters students' projects. This helps them to better choose an adviser whose education and research background is so near to their academic interests.
These professors may also want to apply to get a project, they need a complete CV to introduce themselves to the project owners or investors.