As a developer, I want the ability to generate my pipeline at runtime so that I can customize the stages and steps before execution. The idea being to "run this thing to generate the configuration (or config.yml) instead of hardcoding everything".
A few reasons I think this is useful:
1) Semi dynamic pipelines - not fully dynamic like Jenkins Pipelines, but more flexibility compared to now with what I would say is not much complexity for users
2) Less templating - IMO templating in YAML files is confusing. Thanksfully, there isn't much now but I would be dissapointed.
3) No need to invent additional YAML features. GitLab CI recently introduced the include directive, which I think is a massive mistake.
One of my use cases is to build against multiple different platforms with fanout and fan in when those versions are not hardcoded into the YAML file.