I'd argue that the goal of creating a good, useful changelog can be one of the forcing motivations to establish a working process for commit documentation and great, informative, useful commit messages. If that means git log gardening after the fact, then so be it, but good documentation (of history here) is always worth the work.

The activities of programmer-archaeology are always made so much easier if you have a screed of history for each area of interest in the codebase. Sadly that's still rare, even in groups that do the necessary work to maintain their documentation for the current state of HEAD. There are many times when history matters, is relevant, and those who do not learn it are doomed to repeat it - or at the very least build elaborate theories to explain what happened in the past in order to understand how the present came to be where it is.

The great changelog built on great commit history is probably hard to do outside of a small, focused group. On the other hand, the pull request process has emerged and developers have embraced that level of work, so it feels like it should be achievable.