There are a few reasons why you could find a comment by a user with a rep of 1:
As StoneyB said, the user might have left it as an answer, and it might have been flagged. When this happens, a mod has several options: dismiss the flag and let the answer stand, delete the answer, edit the answer, leave a comment asking the O.P. to elaborate on the answer, or convert the answer to a comment. If the answer is a relevant and insightful comment, it might be converted.
The user may have earned enough rep to leave a comment, and then received enough downvotes that the user's reputation went back down to 1. (This seems unlikely, considering you need a rep of 50 to leave a comment – but it's at least possible in theory.)
The user could be serving a suspension in the penalty box, in which case that user's rep is temporarily set to 1.
As for what you said in your comment to StoneyB:
That comment (originally an answer) does not add anything new to that that can be found from answers given already. So moderators could have simply deleted the answer altogether, instead of converting to a comment.
I agree, it could have gone either way. When making a decision on a borderline case, a few different options come into play. For example, which would seem more welcoming to a new user? Deletion, or conversion?
In this case, the answer already has two downvotes, plus a few comments that seem to go against it. So, perhaps the moderator thought it would be civil to let a dissenting opinion (that is, one that supports the O.P.) be added to the discussion.