- Toolkits and tutorials for inexperienced writers. If you never written a book, you probably don't know first thing you should write is outline. You probably also don't have good templates with typography and layouts that gets instantly recognized as good modern book.
- Taskmaster that keeps nudging you: This is super important role. Book writing is tremendously taxing job for most people and its very easy to get in writer's block, put off things and distracted.
- Regular advances: Almost all publishers would deposit money in your bank account at specific intervals during book writing. This keeps you obligated, at least morally, to keep going.
- Book reviews, editing: Various services like cover design, proof reading, recruiting technical reviewers etc.
- Marketing: Almost all publishers would buy sponsored spots on Amazon as well as Google. Some top-tier publishers would have journalists do book reviews, podcasters invite you for interviews. Some very top-tier publishers would have you do book tours at their expense. However, 90% of the authors will end up doing virtually all promotions themselves.
- Brand recognition: Publishing from very top-tier organizations like MIT Press has huge brand recognition due to their stringent selection process.
I think the future of writing is not even book-size publishing but probably series of posts published as chapters online.
Sure, but you could hire independent specialists with exactly the skills you need rather than relying on your publisher's choices, and still retain overall control of your work and the process of distributing it yourself.
If a publisher provides some degree of overall project management as well then evidently there is also a space for professionals to offer that service independently to those authors who don't wish to get involved in such matters themselves.
Even if you have the patience to do this part, most probably the book design will look amateurish.
Perhaps, but the bookshelf next to me clearly demonstrates that big name publishers are capable of producing poorly designed, crudely illustrated, naively typeset, negligibly edited rubbish too.
And you're going to be doing most of the marketing yourself in any case.
For most people, writing a book is an investment whether you go through a publisher or not. Unless it's just a passion project, you're presumably doing it for your career/brand/etc., not to make a meaningful amount of money. If you are evaluating how to go about a book project based on pure financial ROI, the short answer is you should probably do something else.
- As someone else noted, you can put free eBooks online. If you're writing a book for visibility, being able to distribute free copies (or customized versions of free copies) might be a benefit.
- You don't need to conform to publishing industry length standards. If 75 pages cover the topic, you get most of the benefit of publishing "a regular book" for a lot less effort.
- You're on no one's deadline but your own. This has pros and cons but does let you better work a book around life, work, and other projects.
- The style, format, etc. is all up to you. Again, pros and cons.
I have made (a bit) more money with publishers than self-publishing. I'm also not sure if I'd go with a publisher again barring a strong reason to do so.
You can hire people to do all that -- if you expect to sell your self-published book, and still make more profits than with a publisher.
As for your last remark, most technical publishers are already nimble operations that connect professionals and designers. It just happens that these small companies are owned by large corporations in order to benefit from the access to market channels.
So, unless you're willing to make this a full time job, I don't think it is such a great deal to self-publish your technical books.