Many editors are not fans of agents.
Most editors will not accept unagented manuscripts.
Please help me understand how this makes sense.
I'll respond to this as a post because I think this is an important request.
With the sheer volume of submissions for publication today, agents have become an additional buffer for the overworked (and trust me, they are overworked) editor. If a novel arrives from an agent, it generally means that at the very least, the novel is going to be the correct genre the editor is interested in, is going to be professional and free of spelling and grammatical errors, and have some literary value. For this reason, editors increasingly rely on agents to provide them with quality material as opposed to looking in the slush pile, and major publishing houses have taken on company wide "no unsolicited submissions" policies (meaning, you need an agent) to prevent the mail room from collapsing under the sheer weight of paper. Editors are subject to the policies of their higher-ups, so that means that not every editor may agree with the policy, but they have to follow it.
When I've spoken to editors outside of my work, they've had mixed feelings about agents. Some like them for the reasons above. Some like them because agents are publishing professionals - they get things done quickly and efficiently, and they understand the ins and outs, and they keep the author from calling every week to see how copy-editing is going. A few don't like them because they know the agent will drive up the price of the book, and the more the advance is, the more copies the book must sell to break even or make a profit (most books barely break even). Some authors, on the other hand, are so eager to be published that they'll sign on the dotted line to any amount, whereas the agent needs the advance as a major source of income and wants it to be as high as possible.
The truth is no one knows how much a book is worth until it goes on bookshelves and starts selling, and that's long after the publishing company has invested at least tens of thousands of dollars in it. Some of those costs are non-negotiable - the book is going to cost money to edit and print and ship. The flexible number is the advance, which can range from $250 to several million. The publishing company, perhaps rightly so, feels that the author wins whether the advance is low or high, because if the book is good, it will sell and the author will make money from royalties. The agent and the author know that the book might not sell, and want all the money they can get up front (which is why it's called "an advance" - it's an advance on future royalties) because the advance isn't returned if the book doesn't sell.
Like any business (and publishing is a business), it all comes down to money. That may sound self-serving, but editors and agents and full-time authors have to pay their rent and their bills and their health insurance just like anyone else. Publishing is already an extremely low-paying industry, so everyone fights for what they can get. Oh, and they also want the book to succeed. You have to be fairly altruistic to be in publishing anyway, because the hours are so long and the pay is so low most of the time.
My first book (here's hoping on the next 3 we're currently pitching to different companies) basically sold at a loss. The advance was so ridiculously low that the hours my agent put into contract negotiations means she lost money with her time, as did I in writing it. Even though it will undoubtedly recoup its advance and I'll see royalties, the royalty rate is an industry bottom (7.5%) so the book would have to be a bestseller for me to see any money and for my agent to see her 15% of that money. We agreed to the contract anyway because it was my first book, and other books will hopefully follow it, and the money will slowly increase until it's substantial. It takes a lot of faith, both in the part of my editor and my agent, to maintain me as a client/author. In other words, I'm lucky for every last penny.