I did create a python projet with gnome-builder using the Gnome Application template. I realized that the template builds the entire project structure and adds build capabilities using the meson build system, so I was curious. Why use build system for languages that are not compiled like python?
Build systems aren't only for compiling, they're also for distributing; full applications often include other data like CSS files, UI description files, application metadata files, settings schema files, etc. All these need to be packaged with the application and installed into the right place.
An additional reason is that many applications in a non-compiled language like Python or Javascript sometimes include a private library written in a compiled language, for things that are performance-sensitive.