Quite often, a comment goes with a block of code, and not a single line.
With the current version, the reviewer has to choose 1 line on which the comment will be assigned ; and the reviewee sees the comment in the middle of the code block, at the corresponding line.
When there are several comments on a file, each comment corresponding to a block of code (more than a single line), it's not easy to understand which comment corresponds to which lines.
It would be useful if a comment could be assigned to a block of code (from line X to line Y), and not just to a single line.