Git is more robust than Jira. Git log is accessible offline. Jira descriptions exist only as long as people managing the Jira instance are competent and are migrating the necessary data correctly when migrations are needed. Even migrations from one Jira instance to another (e.g. when companies get acquired and two Jira servers get merged) can be extremely brittle.

It's fair that maybe such simple trivial changes don't deserve such a wordy commit message. But these are just fictional examples that I came up with on the spot. Refactorings, new features, and bugfixes can all have various levels of complexity.

A good commit message helps answer the "why?" questions first and foremost. If a diff is fairly large, pointing out the most important change can be useful. Explanations for non-trivial dataflow can sometimes not make sense in separate documentation, but still be relevant in a commit message.