I think some of the traditional reasons are editing support, marketing support, and the idea of being on real-world bookstore shelves. For the first two, it's

I think some of the traditional reasons are editing support, marketing support, and the idea of being on real-world bookstore shelves. For the first two, it's just a matter of whether you want to/can put in the time to hire out your own editor, do your own marketing, etc. For the last, it comes down to vanity. For tech books especially (but for many other types as well), the world has moved on beyond brick-and-mortar book sales, and the audience you cut off by excluding bookstore relationships is very small. Every time I've done a rational comparison of 'should I keep self-publishing or should I go to a publisher?' I find that for money (not that we're talking 'quit-your-job' levels for 98% of writers), for stress management, and for personal brand building, there is no upside to using a publisher.