When the designers are making the requested revisions to your file, they need to be able to simultaneously view the raw formatting file and your source file. This is so that as they’re making your revisions, they can make sure they’re not introducing anything that wasn’t present in your original manuscript.
If you send an annotated or marked-up PDF rather than a list of corrections, this means that the designer will need to click on and off between several different documents as they work, which can lead to editorial changes done incorrectly or missed altogether. It’s much easier for the designer to have a list which they can print and review, crossing off corrections as they complete them.
A clear and concise list from which they can work also ensures the designer can get the corrections done as expeditiously as possible.