[Jochim]

I think it serves a few potential purposes:

It provides a solid, accurate draft that can be expanded on when targeting non-technical users.

I've often found that lack of visibility leads to messy commits. Surfacing these messages in the changelog introduces an incentive to take more care.

With regards to the usefulness of the descriptions, the associated issue is often linked along with the commit message. This is often omitted in hand-written changelogs. In this case the auto-generated changelog acts as an index, allowing the reader to quickly parse what changed and jump to the associated code or ticket.