The "What's Meta" page says:
On posts tagged feature-request, voting indicates agreement or disagreement with the proposed change rather than just the quality or usefulness of the post itself.
(Emphasis in the original.)
People are voting such posts to indicate whether the feature should be implemented. You can vote like other folks do.
As with all other votes you cast on SE sites, it is not necessary to comment or answer when you vote. My personal guideline as to whether to answer is: a) should the post be answered, and b) has no one already provided the answer I'd give? It is hard to give a comprehensive set of rules regarding what post should be answered but in brief if the post is well-written and the proposal has a fault that the OP is likely to not have foreseen, then it merits an answer.