There are two ways of agreeing with a comment:
- Voting it up (or whatever it's called).
- Commenting in agreement.
Option two is just noise, which leaves option one. Option one helps the good comments, or those people want attention paid to, rise to the top.
When reviewing those queues without this ability I often come across comments that I would like to vote up. In the Reopen and Close queues this is, I think, especially important as people often comment as to why they are voting in the way that they are.
You can signal agreement with someone's reasoning for voting to reopen, without creating additional noise, by voting up their comment. This helps the reasoning for the vote to stand out and provides a different viewpoint to the original closers. The converse is partially true in the Close queue; a user may well have stated why they do not believe a question should be closed. If you agree with their reasoning then being able to vote up the comment makes it more noticeable to potential close voters.
In the Suggested Edit queue the ability is slightly less useful to help explain the reasoning behind something. However, a (significant?) minority of the suggested edits that come through are on new users questions; those who don't necessarily know how to create a good question or how to format it. By enabling voting up of comments people can indicate to newer users what they should be paying attention to.
The ability to vote up comments should be available in every review queue as the point of the queues is to help the community keep the site clean. Voting up of comments enables people to agree with other's users positions, without clutter, and to guide newer users in how to use the site. It can also help keep a question open or re-open a closed question (quite important considering how infrequently it happens).