I love RPG editors and I think you should, too.
Seas of Sand, my perenially-almost-done setting guide, is now finally more or less done, at least in terms of the actual writing and editing process (art's a different beast, as is printing). And let me tell you, my editor (the esteemed Matthew K.) has been 100% absolutely essential throughout the entire thing. Likewise, when I was working on Lowlife and the Big Wet and one or two future projects, having a good editor is amazing. It's fantastic. It makes everything about working on RPG better.
(What does an editor do, you ask? See Jared's guide.)
Here's why editors are great.
Reason #1: you don't need to worry if it's good.
You know how in writing class they always tell you that when you're writing a first draft, the goal is to just put something to paper, and then fix it later? You know how that advice always kind of felt like bullshit, because if it goes to paper bad it's almost worse than having written nothing? Well, with an editor, that's actually not a problem.
The editor's job is, in large part, to make your shit better. You can hand them garbage, and they'll very patiently explain to you why and how it's bad, and the precise changes you need to make to fix it (which very often just means hitting "accept suggestion" in docs).
You can just write. If it's workable, it'll get fixed up; if it's terrible, it'll get cut; if it's great, it'll get polished. But you, the writer, don't have to think about any of that shit while you're writing. You get to just go off.
Reason #2: problem-solvers.
When I hit a wall, like "how do I explain this weird rules concept" or "I'm losing my mind making sure these price tables all match up" or "what the fuck do I put in the next 96 entries on this d100 table?" you can go ask your editor and they will help you. They can do research, or sit with you and drum up ideas, or sift through your manuscript and hunt for issues.
If you're very, very lucky, it's possible your editor will even sometimes just sneakily do a little bit of writing themselves. Fill in a table entry or two, rejig a spell to be way cooler, offer a basically-almost-done monster idea. That's only if you're quite lucky, though—if you expect them to do actual writing, you gotta pay them to do actual writing.
Either way, though, when all hope seems lost and the writer's block comes crashing down, your editor is there to help you solve your problems.
Reason #3: sanity check / hype beast.
If you're like me, you'll sometimes get very deep into a project, or some aspect of a project, and completely lose all sight of home. You bury yourself in the mechanics and ramifications and vast new possibility spaces emerging, and very quickly unmoor yourself, sometimes from the rest of the project entirely. It's difficult to get a sense of whether what you're working on is actually good, since you've just spent 72 hours straight staring straight into it.
At this point, it's your editor's job to come and tell you, frankly, "this is absurd and will never work." They might help you salvage or rework it, but sometimes it turns out what you made is far beyond mortal ken, and thus needs to be cut.
But! Occasionally, you'll spend three days working on something bizarre, and then your editor will come in and tell you that actually, this thing you made is really fucking cool. Then, they push you onwards and help you make it even better.
Your editor is usually both your harshest critic and also your most dedicated cheerleader.
Reason #4: quality control.
Your editor is there to fix all of your dumb mistakes. When I write, my stuff's often filled with stupid little errors. I miss words, I forget punctuation, I garble meanings, I get lost in the weeds. I vary my sentence structure wildly in tables and lists. I'll use a term, thinking I've already defined it, and then never actually leave the definition anywhere. My laid-out drafts have an occasional tendency to repeat paragraphs and titles. Orphans abound.
An editor fixes all that. All of the random tiny issues, they hunt down and solve. It's great.
Reason #5: connections.
Assuming you hire a (semi-)professional RPG editor, one who works in RPGs full time, there's a very good chance they'll know lots of cool people in the space. If you, say, are looking for a layout designer, your editor probably knows somebody; if you need an artist, they probably can help you find one; if you need a distributor, they might have the connect.
(Obviously, this one's a little less true if your editor is totally new to the space—but still.)
While there are lots and lots of RPGs being published all the time, a relatively slim number of those have editors. (A small diatribe: lots of RPGs that list an editor really, in my opinion, have been proofread but not actually edited. Finding typos and errors and fixing unclear bits is valuable, undoubtedly, but fully editing—dev, line, and copy—is a very different beast. One that, I suspect, very few RPG manuscripts properly go through. Once you've gone through a relatively-rigorous editing process, there are lots of little tells to spot on a project that hasn't seen that kind of rigor.)
Anyway, all this to say the number of full-time or close-to-full-time RPG editors is relatively few, and thus most of them know each other. Once you make friends with one editor (probably because you hired them) you'll make friends with more, and thus make friends with lots of people—writers and artists and layout'ers and editors and designers and distributors, the kinds of people who you'll want to work with. The editors I've hired and made friends with have opened many, many doors for me.
Reason #6: good to great.
So here's the thing, right: a lot of what people talk about in regards to editing is in terms of taking something bad and making it good. Or, at least, finding the bad parts and fixing them. That is true, obviously; lots of what editors do is fix mistakes.
Thing is, though, if your manuscript is already good, or if you just do lots of rounds of editing, your editor is there to make your manuscript even better. They take your good ideas, all the stuff that's been percolating in your brain for months, and they juice them up. They play up the good angles, create more game, push the exciting and engaging and interesting.
Writers are often blind about their own work; they've been staring at the pages for too long, they lose the forest for the trees. Editors have their eyes open: they can read through a manuscript and immediately say "ah, these are the parts that work and should be amplified, these parts should be cut." It's those kinds of collaborative, deliberative decisions that can really transform a project from something kind of cool and innovative into something genuinely fantastic.
RPG editors are amazing. Go hire one for your next project.
For what it's worth, I also do RPG editing. (I'm not nearly as talented as many of the ones on your favorite RPGs, but I'm also cheaper and my schedule is probably more open. I also do layout! Double whammy!)