From the help center [bolded mine]:
Voting up a question or answer signals to the rest of the community that a post is interesting, well-researched, and useful, while voting down a post signals the opposite: that the post contains wrong information, is poorly researched, or fails to communicate information. The more that people vote on a post, the more certain future visitors can be of the quality of information contained within that post – not to mention that upvotes are a great way to thank the author of a good post for the time and effort put into writing it!
So while you are correct in stating that there are other means to evaluate the quality of a post, some of them are not immediately visible to other (esp. low-rep) users (like e.g. a flag). Voting down is giving an overt signal to others that there is something wrong with the post. It changes the position of an answer and does not make it look like it was e.g. just as legitimate as another answer just posted.
So while you mention moderation tools, the voting system is first and foremost a tool to signal the quality of a post. Well, it should be - there are a number of exceptions especially in those questions that make it into the "Hot Network Questions".
In other words: The idea is that it is in the responsibility of the community to do both - helping the moderators in moderating the community with whatever tools the reputation threshold is granting and voting according to the quality of the post.
Bonus: A post rapidly voted down is immediately made invisible for all but the highest rep users and is automatically put on the moderators' task list. In other words: Some guys doing this for a hobby [us moderators] do not have to actively intervene and invest time for the post to disappear.