The comment space can also be used to comment publicly on the voting process itself. Naturally it can contain a URL, or several. Within a comment, voters may organize a vote-within-a-vote, adding this up is a trivial data-processing matter which can be performed by any voter (sede can be used for this). Comments are also "undefined bit strings".
The length of the comment is bound to a maximum, set by the vote administration.
Besides the obvious, the comment is a security feature, which prevents vote-spoofing of votes which are expected to yield a predictable content. It makes your vote unique in unpredictable ways.