It is a good substitute.

1. Usually the commit message is often too short to capture the "why" adequately. 2. It is very beneficial to capture the why in one single source of truth, and that usually is not the Git commit message in a business context. Hate on Jira all you want, but if you capture the "why" there, you can add comments, view history, add rich context, link dependencies, add rich context, etc. Can't do that in a Git commit message.

You can put that in the body of the commit message, not everything has to go in the subject line.

My ire is more directed at github PRs than Jira... but the same basic idea applies. You want a single source of truth and you want that as close to the origin (the code) as possible. Your history, dependencies, etc. are all in git already and can be highlighted there if appropriate. For general comments, git notes covers that.

Business (ie. $work) will dictate whatever it wants and that is what get used but for anything I personally have control over, everything goes in the repo itself to prevent platform lock-in. For example, github's been going downhilll lately but all those projects with their history in PRs, etc. now needs to exfiltrate all that data somehow.