The difference is that the Jira ticket is for everyone involved in a project (business analyst, UI designer, QA, support, DBA), while commit messages are written with developers being almost exclusive audience. PRJ-123 might explain why an end user might need it, but the commit message explains why the change (diff) is the way that it is. The ticket answers requirements-level questions, the commit message answers code-level questions. Commit messages are useful both during the review and when a future maintainer is reading the code.

> Additionally, if a change requires multiple commits, you don't want to be repeating the justifications for the entire feature in every commit message. It's redundant. But the commits will all be tied together by the ticket reference in the commit message.

Different commits do different things, so require different justifications. Here's a fictional example to demonstrate:

First commit:

    [PRJ-123] Server: extract class Foo
    
    In the next commit, we're going to need to re-use the foo logic from
    class Bar. Extract new class Foo from Bar to make it available for
    re-use.

Second commit on the same ticket

    [PRJ-123] Server: use Foo in Baz
    
    The users of BazClient need to be able to see foo information in the
    baz dialog.  Include Foo in the data sent by class Baz in the server.

Side note: the user might not even know that they are looking at Foo and Baz, it might be called something else in the UI they are shown. Whether or not this needs to be included in the commit message depends on the situation.

And later in a commit fixing a bug:

    [PRJ-456] Server: check ID for null in Foo
    
    When class Foo was extracted from Bar in commit deadbeef ([PRJ-123]
    Server: extract class Foo, 2026-06-06), a null check for the field
    ID got lost by accident.
    
    Check the field ID for null in class Foo to avoid a
    NullPointerException when a foo event is sent to Baz.