Or, it has been my case, when after a first in-line review, you want to add more coments to that specific chapter. The regular review is a good way to do so.
There are many reason why you may not have provided those extra comments upfront, including, but not limited to: you exchanged ideas with the writer--or somebody else--through other means (messaging, forum posts, etc.) that rang extra bells in your brain about that chapter, you read that chapter again and decided you didn't stress a particular issue enough, the writer made changes to the chapter (without re-publishing it) and you want to comment on the changes, etc.
In such cases, the regular review provides a clean sheet where you can extend yourself at leisure. Of course, it means you're interested in the author, in the story, and you want to go beyond the mechanical act of giving some comments, collecting the points, and move on.
Kiss,
Gacela