Software update wants to install Command Line Tools. Why? I have no Xcode installed
With macOS it is possible to install the command line tools for Xcode seperately from Xcode.

If you did have Xcode installed, the command line tools would be updated with Xcode updates. As you haven't got Xcode installed, these tools are updated from Software Update alongside macOS updates instead.

Installation of the command line tools is triggered by running one of the included programs from the Terminal or via a third party app. The first time that is done, you're asked if you want to install the command line tools, and then everything in that package will be installed.

This is commonly done when installing for example Homebrew, MacPorts, or similar software. It is also used together with lots of open source and other third party software. The command line tools are not only for developing software, but contain many utilities that are of value generally.