It isn't the case for all projects, but for some there are some pre-build steps that the majority of people who want to install and use the project don't have or even know about. The project can put up the source after those steps are done, and the downloader does the final source building and installation with ease.
As an end user looking at the release page, you can't tell the difference between the easy way (the pre-processed source that will work well for you) and the hard way (you'll need to install and invoke several tools for no real benefit).
The underlying problem is that Github make it really easy for people to end up with the hard way, and not the intended easy way. This benefits no-one. I have no problem with them making the repository files available - I just want them to stop confusing people who go to a release page.
Many small projects don't have a releases page. The download zip is just a way of quickly getting the source code for such projects.
To be honest, I wasn't trying to start an argument. I was just pointing out that I like having a "download zip" button.
The specific top level complaint is about the releases page when visited by a human who would have the goal of building and installing the software release. There the "Source Code" link is misleading if the maintainer has a release file that is the expected one for building and installing the software.