It's their job, but sometimes it's easier to argue or prove your point with some clear graphs and objective metrics. I see it as a tool to improve for developers and as an objective view of what really happened.
Sure it helps managers do their jobs, but I think that in some cases we might not even need a manager at all.
You are absolutely right, managers could ask developers for those metrics. It's actually how it's it's done right now, in my experience.
The value I see for developers is to get some objective metrics on a feeling they have. If they think something is inefficient, they should be able to be able experiment with their process. But that needs to be done a bit more objectively.
I think developers could also use this as proof that that the problem is not always with the team but sometimes it may come from management.
The scope creep is a good example of this. A manager keeps adding stuff to the current iteration and the dev team gets swamped. A widget could clearly show the impact of that scope variation.
I see value for the dev team, end-users and management if it's done right.