Not an Answer exists because there are posts that aren't answers, yet don't qualify as spam or offensive. The spam and offensive flags allow 6 common members (15 rep or more) to remove a post, whereas NAA still requires the attention of mods and/or 10k'ers.
Think of three levels of net contribution to the community: a positive contribution, neutral contribution, and a negative contribution.
Any answer post that actually attempts to answer the question is a positive contribution, regardless if it's actually right. Why is it always a net contribution? Well, if it's wrong, people will downvote and comment, which lets you know what not to do.
It should be obvious why spam and offensive post are a negative contribution: they add nothing to the community, and can actually drive others away.
This leaves all the other posts that aren't offensive or spam, yet aren't actual answers in the uncomfortable middle. In some cases, they're comments from users who can't comment, in others, they're similar or follow-up questions. We don't want regular users completely destroying these posts, which is why we hand off responsibility to our elected mods, and the users who have invested enough time into the site to gain 10,000 rep. Often, we want to keep these users around so that they can make an actual contribution down the road, but they just need some guidance. Indiscriminately deleting their posts or downvoting them is not how we encourage future contribution.