You're focusing a bit much on the Developers bit.
It's not just Developers who are working through the code-base.
As a lame example - Incident Response/SRE will also be trying to get their heads around changes being made - especially if they're responding to an outage, and trying to figure out what change broke production - and why it was made.
Not everyone will know every bit of the project as intimately as the Dev team - and having a good commit message will help any unfamiliar response team mitigate, or escalate accordingly.
As a lame example - Incident Response/SRE will also be trying to get their heads around changes being made - especially if they're responding to an outage, and trying to figure out what change broke production - and why it was made.
Not everyone will know every bit of the project as intimately as the Dev team - and having a good commit message will help any unfamiliar response team mitigate, or escalate accordingly.