"Besides being a dream for micromanagers, it seems to be more about signalling progress vs. actually making progress"
Agile is for quick feedback on made progress, to compare it to expected progress and to be able to report your progress and forecast to business people to let them know how far behind the schedule you are as quickly as possible.
Let's say it's January and you need to deliver a product with a 200 use cases on July. After just a few first iterations you will be able to tell if you can deliver all use cases on July or quickly give feedback to business people that you won't be able to. From there you have options: close the project and stop investing time and money in it, add people to the project because you got your info very quickly that you won't be able to make the deadline in 6 months.
Let's say it's January and you need to deliver a product with a 200 use cases on July. After just a few first iterations you will be able to tell if you can deliver all use cases on July or quickly give feedback to business people that you won't be able to. From there you have options: close the project and stop investing time and money in it, add people to the project because you got your info very quickly that you won't be able to make the deadline in 6 months.
Uncle Bob talks about it in depth in one of his lectures here: https://youtu.be/SVRiktFlWxI https://youtu.be/qnq9syXUuFE
And in his book Clean Agile: Back to basics.