They do provide other useful services, like copy-editing and tech review. However, they don't provide these very well, and the amount of money they take in exchange is huge.
They also provide a bit of financing, sort of like how movie studios are basically banks who specialize in investing in one particular area. But with the typical tech book advance coming in under five figures, this isn't very big.
I'll note here that on the Apress book that I worked on, the copy editing and tech review provided were both excellent, certainly far beyond what I would have sourced myself (note "would", not "could" - in the case of tech review I probably knew people who would have been equally good but I'm fairly sure that I would have picked the wrong people).
Only one chapter I wrote got that far, but my experience was that the tech reviewing was at best neutral, whereas the copy editing was ok but not great.
My chapter walked through designing and implementing a custom binary protocol running over UDP for realtime network communication over a LAN. One reviewer suggested that I expand on a bit that went something like, "Create a new project in Xcode" with a step-by-step along the lines of, "1. Open Xcode and pull down the File menu...." This seemed to be well below the level I was aiming for, but of course I could never get anyone at Apress to tell me who they agreed with.
The copy editor was decent but not technically minded, so I had to explain a fair amount. I imagine that technically-minded copy editors are hard to come by, but still, this is a technical press.... The copy editing process was atrocious. I had to download some crazy Adobe software to review the suggested changes, and for some reason they do the indexing before these final changes, so I had to ensure that any alterations didn't reflow pages too much.
When I self published, I ended up crowdsourcing people for these functions by recruiting on my blog and on Twitter. I got a couple of dozen fairly competent people and we were able to divide up the book so that nobody had to do more than a couple of chapters. (Some ended up doing much more, but simply because they wanted to.) That ended up working pretty well, although obviously not every prospective author will have the ability to recruit a couple of dozen helpers like that.
Fair enough, and I'd go further to say: Several of us have access to higher-quality editors and (certainly) tech folks for review than most of the publishers I've dealt with.
Friends of friends mainly. English majors are never very far from professional circles, and several have found that contract tech editing is a way to pay bills while working on items closer to their core.
HN should be a good place for this. If I saw a post asking for editorial help I'd certainly respond, schedule permitting. I'm sure others would as well.
They also provide a bit of financing, sort of like how movie studios are basically banks who specialize in investing in one particular area. But with the typical tech book advance coming in under five figures, this isn't very big.