Let me ask a question right back: Why does an author need/want to show some friends or workshop members the work he's done?
It's not required, sure. But somehow everybody does it anyway. And they listen to what the friends have to say. Because they want and need constructive feedback.
Well, an editor does the same. Except he's more thorough; devoted (by your contract) to your book; he's got a lot more professional experience than your friends do; and so on, and so forth. In this sense, an editor is awesome.
What a lot of people have trouble with is with that advice coming from a position of authority. The editor doesn't merely advise - he can make huge decisions about the book. Starting with whether or not to purchase it.
In the immediate sense - the author doesn't need an editor in authority. Pretty much by definition, if the editor can't convince the author to make a certain change, then forcing the author to make that change can't be in the author's direct benefit (I'm assuming the author has a decent idea of what's good for him; few authors will confess they don't). The editor's authority comes from the publisher, and that's who he's truly serving: the publisher is the one who needs editors to find new authors, to bring the best out of manuscripts, to assure new novels are salable and appropriate to the line, etc. .
The author doesn't need most of that; not directly. The author just needs the system to exist. The author doesn't want an editor to set him in competition with a thousand other MSs - but indirectly he does need him to do that, because otherwise nobody publishes anything. The editor serves as the publisher's sieve, its gate-keeper, its gem-cutter. Does that serve the writers being kept out or shaved off? Not directly - but it upholds the quality of the entire industry, without which nobody would have anything at all.